Building standards guidance - Section 3.3 Flooding and Groundwater: consultation on proposed updates

Draft updated guidance and consultation questions

Draft Section 3.3 Flooding and groundwater

3.3.0 Introduction

Flooding is one of the most severe environmental hazards Scotland faces, with the potential to pose significant risks to buildings and their occupants. Flooding is diverse, often site specific and can occur due to a range of factors.

Sources of flood risk include:

• Watercourse (fluvial)
• Surface water (pluvial)
• Groundwater (including mine water flooding)
• Coastal
• Sewer

It is recognised that extreme weather events, sea level rise and other climate change impacts contribute to increased flooding risks, for example, local pluvial (rainfall) flooding from more frequent short intense rain storms.

In 2018, the Scottish Environment Protection Agency (SEPA) estimated that 1 in 11 homes (around 229,000) across Scotland were at risk of flooding from rivers, surface water and the sea, with projections indicating this would increase to 1 in 9 homes considering a 2080s future climate change scenario^[6]. Over the last decade (2014-2024), the average annual rainfall in Scotland was 10% wetter than the 1961-1990 average, with winters 29% wetter^[7].

Scotland’s National Planning Framework 4 (NPF4) identifies the need to strengthen community resilience to the current and future impacts of climate change by avoiding development at risk of flooding or in a flood risk area as a first principle.

NPF4 flood policy states that development proposals at risk of flooding or in a flood risk area will only be supported in limited circumstances.

The impacts of flooding on a building can include significant damage to materials, services and structure. Contamination from a range of sources is also likely to be present in the floodwater.

Property Flood Resilience (PFR) is a combination of flood resistant and recoverable construction techniques and products when designed into a building lowers its flood risk, it can reduce flood damage and speed up recovery after a flood.

The risk of flooding cannot be removed, however PFR can limit water entering the property, this can reduce damage to the property and shorten time spent out of the property.

Mitigation such as flood alerts, flood protection schemes, PFR measures and sustainable drainage systems (SuDS) can help to reduce the risks and impacts of flooding. In Scotland the responsibility for protecting property from flooding rests with the owner.

Where a new building is sited on land assessed as being at risk of flooding, it is important that flood resistant and recoverable materials and construction methods are used. PFR measures are regarded as an important and cost effective part of a sustainable and proactive approach to flood risk management for new and existing buildings.

When near surface level groundwater is present on a building site there is the potential for construction activity to affect it or for the groundwater to pose a hazard to any new buildings. To reduce the risk to buildings from groundwater, subsoil drainage of a site may be necessary to protect against penetration of groundwater through a building and damage to the building fabric. As noted in the guidance to Standard 3.5, any existing drains that will be affected by the construction of a building should continue to function properly without causing harm to the building, the drain itself or to the health of the occupants.

Conversions – in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

For further guidance on conversions please refer to clause 3.3.2.

Question 1 – Do you have any comments on the amended introductory information setting out background to the issues to be addressed by Standard 3.3?

Yes / No

If you answered ‘Yes’, please provide your comments

3.3.1 Flood risk assessment

A building site flood risk assessment (FRA) should be an integral part of the design and construction process, with the appraisal establishing the likely probability of, and the potential adverse consequences associated with, flooding at the proposed building site, including relevant consideration of associated potential impacts to adjacent land and property.

NPF4 defines ‘at risk of flooding’ as land or built form with an annual probability of being flooded of greater than 0.5%, including an appropriate allowance for future climate change. Areas in Scotland at risk of flooding are indicated on the future flood maps published by SEPA. (Note – surface water (pluvial) flood maps do not yet include a ‘future’ scenario, but this should be available from early 2025)

Site specific flood risk should be evaluated in an FRA, with local flood risk advice being sought from relevant sources such as the local planning authority, SEPA and those responsible for coastal defences.

Whether it is a single or multiple building development, every site is different and each situation may have unique factors, it is crucial to design the FRA to meet the specific building(s) and location characteristics. A FRA should include consideration of the source and type of potential flooding, estimated flood depth, duration and frequency, since these determine the likelihood of being able to keep water out of a building (resistance), and whether it is more cost-effective to plan for water ingress (recoverability). The FRA should also detail flood mitigation options for consideration.

Annex 3.B, Building Standards Advice on Flooding provides guidance and information to consider when addressing flood risk, its source(s) and the effects of flooding on a building.

Guidance on FRA is also provided through Planning Advice note: Flood Risk – planning advice.

SEPA’s Technical Flood Risk Guidance for Stakeholders provides specific requirements for undertaking a FRA in support of a planning application. This document is designed to outline what information SEPA require to be submitted as part of a FRA and methodologies that may be appropriate for design flow estimation (hydrological modelling) and hydraulic modelling.

For site specific FRA, British Standard BS 8533:2017, Assessing and managing flood risk in development – Code of Practice, has been created to help the user to analyse the flood risk of a particular site and to guide the selection of appropriate risk management solutions. It gives recommendations and guidance on the assessment and management of flood risk in developments. It is intended to provide practical assistance for understanding and dealing with the flood risk. Where a FRA has been produced in support of a planning application this should be used as the basis for meeting Standard 3.3 at building warrant stage.

Flood risk assessment for extensions and conversions

Some building work may be permitted development and as such will not be subject to planning approval and related conditions. NPF4 advises that ‘Small scale extensions and alterations to existing buildings will only be supported where they will not significantly increase flood risk’.

Where building work is not subject to planning permission, a FRA should still be undertaken when building in a flood risk area – this would involve construction of new accommodation at or below ground level or conversion of buildings which extend to ground floor level or below. Information on possible sources of flooding and estimated flood levels will prove useful in designing finished floor levels heights and the specification of flood resistant and recoverable materials.

Where there is a known flood risk it is good practice to design PFR into the conversion. The advice within this guidance provides information on how to incorporate PFR measures into a conversion and extension.

Additional consideration should be given where a basement conversion is proposed due to the potentially higher risk and impact of flooding on the building and its occupants.

It is recommended that developers engage with the relevant local authority for specific Planning and flood risk advice.

Surface water run-off to adjacent sites

After the removal of topsoil from a building site, developers should be aware of the risk of possible surface water run-off from the site to adjacent land and properties. It is good practice to have procedures in place to mitigate this occurrence.

Depending on conditions, the formation of channels or small dams to divert the run-off or, where conditions are particularly serious, the installation of field drains or rubble drains may aid the management of runoff from the site, and subsequent disposal.

Question 2 – Do you consider the proposed expansion of the guidance on flood risk assessment to be useful in better framing the action expected and where to access supporting information on undertaking the assessment?

Yes / No

Please provide any comment you have, positive or negative, on the expanded guidance clause.

3.3.2 Groundwater

New building sites should be assessed to establish the existing groundwater level and any fluctuation to the established level brought about by seasonal effect, new construction, excavations or other related activities.

Urbanisation modifies the ‘groundwater cycle’ with marked impacts both during periods of declining aquifer pressures (causing potential land subsidence with building and infrastructure damage) and rising water-table (leading to groundwater flooding). Ground below and immediately adjoining a dwelling that is liable to accumulate groundwater, at a level that could affect the structure or penetrate the building, may require subsoil drainage or other dewatering treatment to mitigate against the harmful effects of such water. An appropriate discharge point should be identified for any groundwater.

The drainage of groundwater may also be necessary for the following reasons:

• to increase the stability of the ground
• to avoid surface flooding where groundwater levels have risen above ground level
• to alleviate subsoil water pressures likely to cause dampness to below-ground accommodation
• to assist in preventing damage to foundations of buildings
• to prevent frost heave of subsoil that could cause fractures to structures such as concrete slabs
• to mitigate surface water flooding – runoff that does not enter a drainage system

The selection of an appropriate drainage layout will depend on the nature of the subsoil and the topography of the ground. Site groundwater tests should be done in accordance with BS 5930. Site soil infiltration tests should be done in accordance with BS EN ISO 22282-5.

Field drains or other measures, where required, should be laid in accordance with the recommendations in BS EN 752.

Question 3 – Do you have any comments on the revised guidance on assessing groundwater risks?

Yes / No

If you answered ‘Yes’, please provide your comments

3.3.3 Resilient construction in flood risk areas

If, following a flood risk assessment a building site has been identified as being at risk of flooding, buildings should be designed and constructed to offer a level of flood resistance and recoverability that can reduce the flood impact on the structure and materials, and mitigate potential risk to life.

Early discussion at design stage with the local authority Building Standards and Planning departments is encouraged for any building site where a risk of flooding is identified.

Property Flood Resilience

Property Flood Resilience (PFR) measures should reduce the amount of water entering buildings (known as resistance measures), or limit the damage caused if water does enter a building (known as recoverability). These measures reduce the threat to the building and the health of the occupants as a result of a flood event.

For new buildings, new build work, conversions and extensions the guidance in this clause will assist both the designer and verifier in establishing what PFR measures should be considered. The guidance also provides designers with useful advice on PFR where an existing property is being repaired after a flood event or installed in anticipation of a possible flood event.

Note that in most cases a building warrant will not be required for repair and remedial works to an existing building following a flood event. Further information on the repair and PFR retrofit of existing buildings can be found within Annex 3.B, Building Standards Advice on Flooding – An introduction to property flood resilience of existing buildings.

Approaches to PFR – Resistance and recoverability

The scale of flood risk based on flood depth, duration and frequency, will determine the likelihood of being able to keep water out of the building (resistance), and whether it is more cost-effective to plan for water ingress (recoverability).

Property flood resilience - combine both resistance and recoverability measures that can be incorporated into the building fabric and / or fixtures (this would not include items such as ‘white goods’) and fittings so as to reduce the consequences of flood water entering the property.

Flood resistance - construction of a building in such a way as to prevent or minimize flood water entering the building and damaging its fabric. The depth at which resistance measures can be used should be based on advice from a construction professional such as a structural engineer. The use of resistance above the design flood depth may be limited to certain forms of construction.

Flood recoverability - Use of materials, products and building methods that prevent the internal fabric of the building from being unduly damaged by floodwater and allow it to recover quickly after the flood.

Flood risk design assessment

The guidance within this clause will be most effective in conjunction with the recommendations in clause 3.3.1 Flood risk assessment. The assessment should consider the potential risks and what is the most appropriate PFR for the building type, its use and the end user. Depending on the nature and severity of the risks identified such as depth and source of flooding, specialist advice may be required.

Supporting guidance

The recommendations in this clause are supported by guidance in BS 85500: 2025 – Flood resistant and resilient construction - guide to improving the flood performance of buildings and the CIRIA Code of Practice (CoP) for property flood resilience C790.

BS 85500: 2025, is intended to help identify when flood resilient construction is appropriate and to give guidance on achieving this. This standard is applicable to new buildings, extensions and the retrofitting of existing properties.

The CIRIA Code of Practice is concerned with physical measures that can be introduced to buildings at risk from flooding. The CoP for property flood resilience (C790) sets out six standards specifying what should be achieved when delivering PFR. These standards are supported by guidance on how the standards should be met by following stages within a process in CIRIA C790B. These standards provide a benchmark for good practice to support the consistent and effective implementation of PFR.

CIRIA Code of Practice Standards

1. Hazard assessment:

This standard shall deliver a property level flood risk assessment, which clearly summarises the available hazard information to determine the likelihood and severity of flooding from different sources. This information will be used to inform the selection and design of PFR measures

2. Property survey¹:

This standard requires that an appropriate survey of the property and end user requirements is carried out. The purpose of the survey is to assess the current level of flood resilience of a property to provide the necessary information for the identification of the PFR options suitable for the property. Each building at the property shall be surveyed.

3. Options development and design:

This standard allows options for PFR to be identified and considered. These options shall reflect the outcomes from standards 1 and 2, and PFR measures suitable for the property and specify the most suitable PFR measure for the property. The options for PFR will consider the use of:

• Measures that restrict water entry to the building under defined conditions
• Materials that are recoverable after water contact
• Services, fixtures and fittings that are recoverable by their location and/or ability to resist water damage.
• The design specification will be based on the information provided by Standard 1 and 2, and the measures specified shall be selected impartially.

4. Construction:

This standard ensures that the construction works deliver the specified PFR measures to the required standard and with the desired outcomes.

5. Commissioning and handover:

This standard ensures that the completed PFR construction work will operate effectively as designed, and that the end user has all relevant information and has been instructed in any deployment, operation and maintenance requirements.

6. Operation and maintenance²:

This standard ensures that the completed PFR construction works are properly operated and maintained, and that any demountable equipment is stored correctly. Note that the responsibilities and duties for operation and maintenance are defined as part of meeting the requirements of Standard 5.

Notes:

1. The survey of an existing property in relation to PFR and a building warrant application is not required.

2. Standard 6 is beyond the scope of the building regulations.

The supporting guidance explains how the standards can be met and how the six staged process aligned with the standards complies with the CoP and follows good practice.

Flood recoverable construction

The use of flood recoverable materials and forms of construction can minimise the damage done by flood water, and reduce the amount of time required to recover following a flood event, but will not be sufficient to make a development acceptable when the probability of flooding indicates that it should not be permitted by the planning process. Flood resilient materials and techniques are most likely applicable for alterations, conversions and small scale extensions and for redevelopment within built-up areas where there is a risk of flooding.

Where a flood protection scheme exists or is anticipated, PFR may still be appropriate at a property protection level. The retrofit of the PFR at a property, either as part of a refurbishment or post flood event reinstatement works should be considered.

Foundations and substructure

Foundation and substructure design should take into account the predicted flood event, ground conditions and existing water table levels. Design elements such as foundation type, depth and concrete specification should be considered.

Some foundation types may not be suitable for certain sources of flooding, specialist design advice should be considered.

Basements

Basements in new buildings and conversions of existing basement areas are not recommended for habitable use in flood risk areas due to their vulnerability to flooding through openings and overtopping.

Where basements are proposed these should be designed to BS 8102: 2022 - Protection of below ground structures against water ingress.

Lower ground floors also require careful design detailing and material selection to limit external water levels and moisture.

Floor construction

Where a flood risk is identified the floors should be designed to resist both hydrostatic and hydrodynamic ground pressure and impacts, in addition to restricting water entering the building. All floors should be designed to withstand design flood levels and pressures. See Table 3.3 for floor construction type.

Floor Construction & Flood resilience

Ground supported concrete slab: most suitable

Suspended Concrete floor: suitable

Suspended timber floor, sealed, use of marine plywood: suitable

Suspended timber floor and chipboard flooring: unsuitable

Ground supported concrete floors

In flood risk areas solid concrete floors are generally considered more resilient because they prevent water accumulation beneath the floor level. The concrete slab should be a minimum 150 mm with the damp-proof membrane fully sealed and connected to the damp-proof course rather than over lapped and tapped. A dense concrete ground supported floor should provide resistance to water ingress from the ground and also be recoverable.

Flood resilient insulation should be used to insulate the floor. It may be advisable to raise ground floor levels above the design flood levels.

However, Standard 4.1 Access to buildings, requires to be met and this may prohibit raised ground levels due to the need for accessible ramps.

Recoverable elements

Insulation and underfloor heating should be placed on top of the slab to prevent damage in the event of a flood. When specifying insulation, consideration should be given to not only thermal performance but the product’s recoverability after a flood event.

Screeds

Sand cement screeds are generally resilient to the effect of water. When specifying a screed for flood resilience, certain elements needs to be considered: what is the screed made of, its thickness, what functions is it required to fulfil, water absorption levels and time to dry out.

Concrete suspended floors

Suspended floor construction is not recommended for ground floors built below the predicted flood level. Water, mud and flood debris can become lodged within the subfloor ventilated space.

Where a suspended floor construction is proposed, a concrete suspended floor will be more suitable than timber due to its recoverable nature. The location and design of sub floor ventilation should be considered as this can provide a potential route for water ingress. Periscopic vents that terminate above the design flood level should be considered.

Recoverable elements

Insulation and underfloor heating should be placed on top of the floor to prevent damage in the event of a flood. When specifying insulation consideration should be given to not only thermal performance but the recoverability of the product after a flood event.

Suspended timber floors

Suspended timber floors are not typically suitable in a flood risk area.

However, where a timber floor is proposed then recoverability is the key consideration for any design. Timber joists can be sealed and treated with preservative to ensure they are more water resistant and to permit them to dry out faster after having been immersed in floodwater. Coverings such as softwood floorboards or chipboard flooring are not suitable.

If it is not possible to place the insulation above the joists on the ‘dry side’ of the floor, then the specified insulation should be able to maintain its position, thermal and structural integrity in a flood event. Flood resilient insulation in either case will provide flood resilience.

As the ventilated space in a suspended floor may fill with flood water, it will be difficult to drain and dry out. A possible solution to this problem could be the use of a sump pump installed to drain away trapped water. An alternative to a sump pump is for the sub floor to be sloped to a low point where a water tight hatch can be opened to allow the water to flow out.

Although not part of the building regulations floor coverings should be given careful consideration. Floor finishings such as ceramic tiles and hardwoods may be considered recoverable options.

External wall construction & Flood resilience

Solid concrete wall¹: suitable

2 leaves of masonry: suitable

Masonry and timber kit: least suitable

Notes:

1. Solid concrete walls need only be built to above the design flood level, from there other forms of construction can be used.

Careful consideration should be given to external walls and the junctions they form with floors and openings. The correct detailing and material selection of these element will assist in making the dwelling more resistant and recoverable to flooding.

Where a flood risk is identified the walls should be designed to resist both hydrostatic and hydrodynamic pressure and impacts, in addition to restricting water entering the building.

Service penetrations and cavity vents in the wall should also be considered as routes for water ingress. If possible these should be placed above the design flood level, plus an appropriate freeboard allowance.

When specifying wall insulation, careful consideration should be given to ensure that it is both resistant and recoverable in the event of a flood.

Wall construction (solid masonry)

Concrete walls

Solid concrete walls designed to suit proposed flood levels, loads and conditions are a suitable solution in a high flood risk area. Careful consideration is needed around jointing and junctions to ensure minimum water penetration in a flood event.

Masonry walls

Masonry walls can provide an effective method of minimising water penetration, however this is dependent on the correct choice of brick/block and mortar.

Brick, blockwork and mortar with a low water porosity should be used the design flood level, plus an appropriate freeboard allowance.

Where the external masonry wall has a cavity the following elements require careful consideration.

• The construction type of the internal leaf – a masonry inner leaf is preferable to a timber kit construction;
• The location and design of weep holes, perpend vents and service penetration to minimise the cavity flooding;
• If the cavity is insulated the type specified should be flood recoverable, will not slump and be able to maintain its structural integrity and thermal performance.

Timber frame walls

Timber framed walls provided less flood resistance and recoverability in the event of a flood. However, where a timber kit is proposed then consideration should be given to using flood recoverable materials. Elements such sheathing, insulation and plasterboard should be flood recoverable. The timber kit can be sheeted so that the plasterboard runs horizontally from ‘east to west’ rather than the normal vertical ‘north to south’. This allows for easier replacement in the event of a flood. (designers should ensure that any required fire resistance performance is not affected by altering the plane of the plasterboard sheets)

Consideration should also be given to treating the timber kit with preservatives to ensure they are water resistant.

Separating walls

Separating walls should also be considered as a potential routes for flooding from one property to another. Resistance and recoverability of fire, thermal and sound insulation materials should be considered. It is preferable to build a solid masonry separating wall, however where a cavity wall is required for the purpose of thermal insulation then that insulation should meet the recommendation set out in the previous paragraphs.

Internal partition walls

Internal walls should be constructed using flood recoverable materials and ideally not have any cavities. Services such as electricity and plumbing can be run behind a service void which sits on top of the resistant internal wall lining.

Masonry internal walls offer the most resilient from of construction for buildings. However, timber stud walls on the ground floor can be sheeted so that the

plasterboard runs horizontally from ‘east to west’ rather than the normal vertical ‘north to south’. This allows for easier replacement in the event of a flood. Elements such sound and fire insulation should be of a type that is flood resilient.

Internal linings

Internal linings can be either sacrificial or flood resilient. Where sacrificial linings such as Gypsum plasterboard is proposed it is recommended that they are fixed as noted above.

Internal stairs

Ground floor stairs located within the predicted flood level should be designed to be recoverable in the event of a flood.

Options for recoverability of stairs include:

• Make the bottom treads and risers out of a flood recoverable material and construct the enclosure of the void / cupboard to allow for drying and decontamination
• Make the bottom treads and risers out of concrete, which will eliminate the void and therefore eliminating the need for drying and decontamination

Doors, windows and subfloor air vents

Doors, windows and subfloor air vents are the main point of water ingress into a building. The location of these openings should be considered to help mitigate the potential flood risk and water ingress. Resistant and recoverable materials and products should be used to limit water ingress into the building, reduce potential damage and speed up recovery time post flood event.

External doors

If possible, raising the threshold of the entrance of the property to above the design flood level is the simplest way of reducing the risk of water entering the building. However, Standard 4.1 Access to buildings, requires to be met and this may prohibit raised thresholds due to the need for accessible ramps.

External flood doors should be to BS 851188 and be fitted and installed as per the manufacturer’s instructions. Careful consideration should be given to the detailing and sealing around the junction between the door frame and walls to ensure a tight fit and no unintended voids or cavities.

Windows and glazing

Windows and glazing below the predicted design flood level should be avoided. Where this is not possible these then the windows and glazing should be flood resilient and designed to BS 851188 and installed as per the manufacturer’s instructions. The detailing and sealing around window / glazing openings should carefully considered.

Where windows and glazing are below the predicted design flood level consideration should be given to the type of flood, its depth and the possible water pressure that may result in failure of the glass and sudden inundation of flood water. The impact on the glass from debris in flowing water should also be considered.

Internal doors

Internal doors should be made of flood resilient materials.

Another option is the use of lightweight internal doors on rising butt hinges. Where advance warning and time allows they can be removed and placed in a dry location. Note that anywhere a fire door is required, such as in a protected enclosure, entrance door to a flat or a door between a dwelling and an integral garage will precluded the use of lightweight removable internal doors.

Subfloor air vents

The location of sub-floor air vents below the predicted design flood level should be avoided if possible.

All sub-floor air vents below this level should be flood resilient and in accordance with BS 851188.

Another option is the use of telescopic vents which terminate above the predicted design flood level.

Question 4 – A significant expansion on previous guidance on flood resilient construction is proposed. Do you have any views on the usefulness of this additional information, including example construction details?

Yes / No

If you answered ‘Yes’, please provide your views and any relevant supporting information.

Question 5 – Are there additional construction details or other useful information which could also be included in clause 3.3.3?

Yes / No

If you answered ‘Yes’, please describe the additional construction details or information you consider would be useful to include in clause 3.3.3.

General feedback

Question 6 – Having reviewed the proposed changes in the context of current guidance to Standard 3.3, do you agree there is a need to update the guidance in Section 3.3 (Flooding and groundwater) of the Technical Handbooks?

Yes / No

Please provide information on why you agree or disagree.

Question 7 – Does the revised guidance provide enough information to understand what is required to achieve compliance with Mandatory Standard 3.3, Flooding and groundwater?

“Standard 3.3 Every building must be designed and constructed in such a way that there will not be a threat to the building or the health of occupants as a result of flooding and the accumulation of groundwater.”

Yes / No

Please provide any additional comments or information.

Question 8 – Do you consider that the revised guidance will add to the potential cost of development?

Yes / No

If ‘Yes’, please set out your reasoning and provide any information you consider relevant on potential costs.