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

Annex 3.B, Building Standards Advice on Flooding

3.B.0 Introduction

The building standards system in Scotland is intended to ensure that building work on both new and existing buildings is compliant with the mandatory functional standards. Compliance with the standards can be met by following the guidance set out within the Scottish Building Standards Technical Handbooks. The system also has flexibility, allowing compliance to be achieved by solutions other than those outlined in the Technical Handbook guidance. The main text in this Annex is taken from the Flood Risk Planning Advice Note (PAN) 69, where it relates to building standards. It has been reviewed and updated to support the guidance in Section 3.3 with the aim of providing further information on flooding and how it affects buildings. Additional flood guidance and advice can also be found in documents such as BS 85500: 2025 and CIRIA Code of Practice.

NPF4 has a presumption against development in areas at risk of flooding. Local authorities and developers have a responsibility to ensure that future building sites avoid areas at flood risk as a first principle. However, there are circumstances where development would benefit from selecting designs, forms of construction and materials which may help to minimise the risks and impacts of a flood event on buildings.

3.B.1 Purpose of This Document

The Annex sets out background information on the water environment and the factors which contribute to flooding. This includes watercourses, coasts, sewer surcharging, groundwater, and the influence of climate change. It provides background information on the impact of floodwater on buildings, and advice on flood resistant and recoverable materials and forms of construction that may be required to demonstrate compliance with Standard 3.3.

The key purpose is to supplement the guidance in Section 3.3 by raising awareness and knowledge of flood risk and measures to mitigate flooding. Further information on managing flood risk can be found on the Scottish Government’s water policy webpage.

The Annex describes, in more detail, what should be considered by applicants and designers prior to the submission of a building warrant application in a flood risk area. It provides more information on the types of flood risk and the effects of flooding on a building. It also provides information on drainage assessments and how relevant clauses within the Technical Handbooks relate to drainage assessment.

A good practice guide has been developed for Building Standards verifiers when assessing building warrant applications with flood risk. It provides advice (flow chart) on how building standards verifiers can interact with other sections within the local authority who have a role in flood risk assessment and mitigation such as Planning and Environmental Health.

The use of this document does not remove the need to obtain a building warrant where it is required by the building regulations. Furthermore, it is acceptable to use alternative methods of showing that compliance with the building standards has been or will be achieved.

3.B.2 Sources of flooding

Watercourse (Fluvial)

The principal cause of watercourse flooding is excessive rainfall or snow melt within a limited period, which overwhelms the natural drainage capacity, particularly when the ground is already saturated. Floods can also occur during lesser events e.g. when river channels become blocked with debris, watercourses which are culverted or pass under bridges being the most vulnerable, and in the event of a structural failure of defences. Some areas are subject to combinations of tidal and watercourse impacts.

The impacts of flooding vary at different locations. For example, flooding of agricultural land can be costly to the individual farmer, but is unlikely to involve a serious threat to human life. However, the potential overtopping and possible failure of a high flood bank defending a densely populated area presents a greater threat to life and property. Rapid flows due to flash flooding following failure of defences pose a greater risk to life than a steady rise in water level.

The impacts of watercourse flooding can be aggravated by:

• the growth of built development in catchments and other changes in land use, which increase the rate and volume of run-off and reduce floodplain capacity;
• sediment deposition that has changed river cross-sections and affected
• channel capacity particularly in culverted watercourses;
• lack of maintenance of flood defence systems, watercourses and culverts particularly where this leads to channel blockage;
• canalisation, modification and diversion of rivers, which increase the rate of flow and decrease the time taken for water to travel within a catchment; and
• building of structures (e.g. embankments) which restrict flows over historical flood plains and thereby create additional flood risks both upstream and downstream.

Surface water (Pluvial)

Surface waterflooding occurs after periods of heavy rainfall. Rainwater ponds or flows over the ground (overland flow) before it enters a natural or man-made drainage systems (e.g. a river or sewer/drain). This may be due to a range of reasons including blocked drains or even rainwater running off roads. It can also occur when drainage systems are at full capacity. It is often combined with sewer flooding and groundwater flooding.

Surface water flood maps are currently available in the UK, but as areas change and develop, this may change where water could collect. There are specific SEPA maps for Scotland that can be referred to via the link.

Sewer

Sewer flooding occurs when the sewerage infrastructure has to deal with loads beyond its design capacity. This occurs most often as a result of high intensity rainfall events.

The sewerage infrastructure in many parts of Scotland is an old combined system, taking both foul sewage and surface water. In many places it is of limited capacity. This means that when there is heavy rainfall, the system may be unable to deal with the volume of water, which can back up causing spills of foul sewage through WCs and manhole covers.

This has also resulted in situations where combined sewer overflows, designed to relieve pressure from excessive surface water, may discharge into watercourses and have the potential to affect flooded areas.

Other factors can contribute to sewer flooding, such as blockages, illegal connections to the public sewer system, or infiltration of surface water. The latter two can contribute flows beyond the capacity of the sewer.

Groundwater

Groundwater flooding occurs when the water table rises above ground level. In Scotland this is most commonly associated with the movement of water through sands and gravels, often connected to the rise and fall of river levels.

Groundwater flooding can affect homes and businesses in the UK although this is not the most common type of flooding. For groundwater flooding to occur, the water table in an area must rise as a result of increased rain. When this water table rises up through a slope, there may be a point at which the water table is above the ground level. If this happens, the water will flow over the surface as it cannot seep into the ground – this is groundwater flooding.

Unlike other types of flooding, groundwater flooding may require you to consider measures to protect your home that will prevent water from rising up from below your building – such as a floor membrane.

Coastal

Inundation by the sea is largely due to combinations of high tide, storm surge and wave activity raising the level of the sea above adjoining land.

For the coast, tide-tables^[8] give predictions of astronomical tides and take into account seasonal average weather conditions for the locality.

However meteorological conditions can cause storm surges and if in phase with the normal tidal cycle, can result in levels considerably higher than those predicted by reference to tide-tables. Storm surges are associated with intense depressions and in most years, several surges of 1 to 1.5 metres are experienced and 2 metre surges are not uncommon.

An additional hazard in coastal flooding is the height of waves which over a long fetch can add considerably to the level of the water. Fortunately, the extremes of tide, surge and wave rarely coincide. Coastal flood can also bring with it damage to buildings caused by impact from trees and other debris. Climate change driven sea level rise are also causing an increase in flood risk.

Building warrant applications where there may be a flood risk

In addition to demonstrating compliance with Standard 3.3 flooding and ground water and 3.4 Moisture from the ground, a drainage assessment of the site may be required where flooding has been identified as a risk.

Drainage Assessment

When flooding is an issue the provision of drainage is unlikely to be straightforward and a drainage assessment may be required. NPF4 advises that “Development proposals will:

i. not increase the risk of surface water flooding to others, or itself be at risk.

ii. manage all rain and surface water through sustainable drainage systems (SuDS), which should form part of and integrate with proposed and exiting blue-green infrastructure. All proposals should presume no surface water connection to the combined sewer;

iii. Seek to minimise the area of impermeable surface”

The assessment should demonstrate that the proposal would have a neutral or better effect on the risk of flooding.

Planning authorities have a duty to consult Scottish Water and SEPA on appropriate planning applications, and where drainage is a significant issue, applicants should provide a drainage assessment as part of their supporting material.

Drainage assessments in Scotland are tailored to the circumstances of each site and proposed development. The issues typically addressed include:

• Brief description of the site and location;
• Pre-development foul and surface water drainage provision, including field drains;
• Surface water calculations;
• Surface water disposal, including SuDS design and flow attenuation;
• Foul drainage design proposal and standards;
• Maintenance regime;
• Any agreements reached with Scottish Water and SEPA.

The potential of garden ground and other open space to become waterlogged or suffer from localised flooding is something to be considered from the earliest stages of a site investigation. The problem may not be obvious from a site inspection as it often occurs intermittently, possibly on a seasonal basis or when rainfall is intense. It may be related to local fluctuations in the water table. Problems can arise or be made worse if the construction process involves re-grading, disturbing the soil profile or compacting the ground with heavy plant. The layout of buildings can also be a contributory factor because foundations can impede the flow of sub-surface water. Note that Scottish Water will not accept discharge of ground water to the sewer network.

For large developments, proposals in areas where drainage is constrained or otherwise problematic, and where building works may affect drainage off-site, it is good practice for a drainage assessment (also referred to as drainage impact assessment) to be submitted with the planning application. The purpose is to assess the potential for flood risk and pollution, and show that a satisfactory means of waste and surface water drainage can be provided. Planning authorities may attach conditions but the developer has the primary responsibility for ensuring that the land and development are fit for purpose.

A building warrant is also required prior to construction of surface water or wastewater drainage work serving a building and it is recommended that an early assessment of options is made at the inception of the project. Building regulations cover work within the curtilage of a building only but drainage systems can impact on a much wider area. Designers should be aware of implications to the environment as a whole, when designing drainage systems.

Effects of floodwater on buildings

Floodwater can penetrate buildings rapidly, causing widespread damage to floors, walls, finishes and services, and structural damage in more severe floods. The vulnerability of individual buildings is dependent on construction methods and building materials. For example, poor construction techniques and some common bricks are permeable allowing water to penetrate quickly to the building interior. Inside the building, gypsum based plasters (e.g. most plasterboard) absorb large quantities of water and distort within minutes of contact with water. Even with measures to flood proof buildings, water will tend to find its way through weak points within the wall such as cracks and voids in the mortar jointing, brickwork or rendering. For semi-detached and terraced houses floodwater may also seep through party walls with neighbouring properties, above or below floor level.

The processes and pathways by which water enters a building during a flood depend on the characteristics of the flood – specifically flood depth and duration, and water velocity. Groundwater flooding results in water entering cellars, basements and voids beneath floors causing problems of damp in walls. In general terms:

• Shallow floods will penetrate “weak” points in the building such as air vents and cracks in brickwork, and will overtop doorsteps. The use of temporary flood barriers or proprietary flood proofing systems will assist in restricting the amount of flood water penetration.
• Deeper floods and faster flowing water are likely to penetrate the structure of buildings more quickly. Floodwater will enter buildings through a larger number of pathways including drainage pipes from downstairs toilets and baths and even windows that may be broken due to the pressure of water or debris.
• The location of service penetrations from broadband and telecoms should be considered along with the location of mechanical services such as air source heat pumps and micro renewable services.
• Where flood depths exceed 300 mm there is a risk of structural damage and collapse, particularly if the water exerts pressure on only one side of a wall. If the building is being designed to withstand pressure from a depth greater than 300 mm of resistance, a structural engineer should engaged.

Secondary effects of flooding

There are also secondary effects of floodwater on building structure and the health of the occupants. These impacts include:

• Contamination by sewage and the sediments from both watercourses and blocked drains. Watercourse, coastal and sewer flooding can lead to the contamination of flooded properties. In the case of sewer flooding, raw sewage can be deposited on affected sites. Following a flood, external walls will be dirty and may be permanently stained if not cleaned. Contaminated sediments may be deposited on site and these must be removed.
• Damp conditions following a flood may lead to the growth of moulds that can damage the building and present a health hazard. Buildings with excess moisture, poor ventilation and those exposed to standing floodwater can be breeding grounds for moulds. All moulds have the potential to cause health impacts, such as mild to severe allergic reactions and breathing difficulties for asthmatics.
• Coastal flooding can lead to salt water damage such as the corrosion of metal fittings including metal ducting and switch boxes, and steel reinforcement within reinforced concrete.
• Flood damage can also result from the impact of debris, corrosion due to chemical contaminants, changing hydrostatic pressure due to waves, pressure from breaking waves, lift due to the buoyancy of the property and scour undermining the foundations.

Summary of the potential effects of exposure to floodwater

Masonry, Concrete and Brick

In general, masonry and concrete are unlikely to be severely damaged by contact with floodwater. In the case of coastal flooding, salt water may cause surface powdering and flaking of soft brickwork. Lightweight concrete may expand and contract depending on moisture content so wetting and drying may cause some cracking.

Timber

Timber swells and may distort on wetting. In timber framed buildings, swelling of immersed members could cause damage in other parts of the structure, e.g. through stresses on external cladding. Timbers that become wet and cannot dry may be at risk of decay in the long term. Guidance on the selection of timber preservatives is provided in BS5268: Part 5:1989 for structural timber and BS 1186 Part1:1991 for joinery.

Wall finishes

Renderings containing cement are unlikely to suffer damage. Lime based plasters are preferable to gypsum which softens when wet. Similarly, following flooding, any plasterboard will probably be damaged beyond repair and require to be removed and replaced

Metals

Metals are affected by the corrosive effects of sea water so resistant metals rather than mild steel should be used in coastal areas where flood risk is an issue.

Insulation

Flood resilient insulation will not absorb water but may restrict drying out of a cavity wall. Mineral fibre and other absorptive insulants will retain water and can lose their insulating properties or disintegrate over time.

Construction techniques

There are two basic approaches that may be appropriate for the protection of buildings against the effects of flooding:

• Resistance aims to prevent floodwater from entering a building. It relies on the use of waterproof barriers integral to the structure, across entrances and non-return valves on drains. Making buildings resistant however can be difficult and is unlikely to work if buildings are subject to flooding for long periods. Simple measures are unlikely to prevent water penetration for more than a few hours while more complex solutions may protect the building for a day or two.
• Recoverability assumes water will enter the building and is based on the use of water resistant and resilient materials within the building and the raising of services such as air source heat pumps, electrical wiring and sockets above the maximum flood level. This is the most practical approach and there are a number of ways to limit the damage from flooding. Recoverable construction should also allow water to drain easily from the building following a flood and not retain it in walls, floors and air pockets within the building footprint.

Building Components

Component: Floor construction

Most suitable – Ground supported concrete, pre-cast or in situ slab

Suitable – Suspended concrete slab/beam and block floor

Least Suitable – Timber floor joists, fully sealed, use of marine plywood

Unsuitable – Untreated timber joist and chipboard flooring

Component: Floor covering

Most suitable – Clay tiles, rubber sheet floors, vinyl sheet floors

Suitable – Vinyl tiles, ceramic tiles, hardwood

Least Suitable – Laminate flooring

Unsuitable – Carpet, rugs

Component: External walls (to maximum flood level)

Most suitable – Engineering brick, reinforced concrete

Suitable – Low absorption brick <3%

Least Suitable – High absorption facing brick

Unsuitable – Timber frame and cladding

Component: External doors

Most suitable – Flood doors to BS 851188, solid panels with waterproof adhesives, aluminium, plastic or steel

Suitable – Epoxy sealed doors

Least Suitable – Unsealed timber doors

Unsuitable – Hollow-core plywood doors

Component: Internal doors

Most suitable – Doors made with flood resilient materials

Suitable – Lightweight doors with rising butt (to be removed on threat of flooding)

Least Suitable – Sealed solid timber doors

Unsuitable – Hollow-core plywood doors

Component: Insulation

Most suitable – Rigid flood resilient insulation

Suitable – Reflective insulation

Least Suitable - Unsuitable – Mineral wool insulation

Component: Windows

Most suitable – Plastic, metal

Suitable – Epoxy sealed timber with waterproof glues and steel or brass fittings

Least Suitable – Timber with PVA glues and mild steel fittings

Unsuitable -

Building and Flooding Checklist

Factors to consider before building in areas where flood risk is an issue

1. Background information

Is the development in a flood risk area:

Is the source of floodwater from watercourses, coastal waters, groundwater or sewers?

If the source of flooding is from watercourses or coastal waters, is the annual probability of flooding greater than 0.5% (sometimes referred to as 1 in 200 YR)which must include an appropriate allowance for future climate change.

What is the maximum flood level in the 0.5% event, which must include an appropriate allowance for future climate change?

Would the site be inundated rapidly, for example due to a breach in a flood

defence, or slowly, for example in the case of groundwater flooding?

2. General issues (adapted from ABI, Assessment of the cost and effect on future claims of installing flood damage resistant measures)

Records of previous flood levels.

Ground conditions – e.g. permeability and provision of field drains.

Floodwater pathways into the building – e.g. poorly maintained masonry, ventilation grilles, doors.

3. Floors

Where possible use dense concrete screeds on solid concrete floor slabs.

Use treated timber to protect it from rotting if exposed to standing water.

Use steel joists and wall plates rather than timber.

Use a damp proofing material around the ends of floor joists.

Use a sump and pumping system in buildings at risk of groundwater flooding.

Replace expensive flooring, such as oak floorboards with treated timber boards.

Raise floor levels.

4. Walls

Install air bricks above expected flood level and duct down to solum.

Use close cell insulation.

Replace gypsum plaster with more water resistant materials.

Fix plasterboard horizontally.

5. Interiors

Replace door hinges with butt hinges that allow door to be removed and placed in a dry area prior to a flood.

Fit kitchen units with extendable plastic or stainless steel feet so that they will not be damaged by shallow flooding.

Use raised fitted ovens and fit above highest expected flood level.

6. Services

Move service meters at least 1 metre (preferably more) above the 0.5% flood level, which must include an appropriate allowance for future climate change.

Consider completing electrical wiring from the first floor of the property so it will not require replacement if the property is flooded.

Can non-return valves be placed in drainage pipes to prevent water backing up pipes into the building?

If a sump pump is proposed, is a back-up power supply appropriate in case of power failure?

Level access and raising floor levels

There has been a requirement in the building regulations since 1985 for all new buildings, other than dwellings, to be accessible to disabled people.

Building Standard 4.1 States that:

‘Every building must be designed and constructed in such a way that all occupants and visitors are provided with safe, convenient and unassisted means of access to the building’.

The Building Standards Technical Handbooks provide guidance on access issues and include an example of what might be termed reasonably practicable in such cases.

There is no reason why a building designed with level or ramped access should be any more susceptible to flooding than one with stepped access. However, careful consideration should be given to appropriate detailing of damp-proofing, weather-proofing and drainage, particularly on and around an accessible entrance. For both domestic and non-domestic developments of all scales, it is vital that this matter is considered as early as is practicable in the design process as it will greatly influence both site layout and ground levels.

Where there are conflicts between the need for level access and the flood risk, this should be discussed with planning and building standards verifier at an early stage. Where land raising is proposed, in order to achieve level access to dwellings it may have an impact on floodplain capacity.

Other building design considerations

The following Building Standards, although not written with flooding in mind, will have a positive influence on how buildings react after flooding has occurred and may therefore be relevant:

Building Standard 3.10 states:

‘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 the occupants as a result of moisture from precipitation to the inner face of the building’.

A floor, wall, roof or other building element exposed to precipitation, or wind driven moisture, should prevent penetration of moisture to the inner surface of any part of a dwelling so as to protect the occupants and to ensure that the building is not damaged. For external wall constructions it is important that the wall is designed and constructed to suit the degree of exposure to wind and rain that it may be subject to.

Building Standard 3.14 states:

‘Every building must be designed and constructed in such a way that the air quality inside the building is not a threat to the hygiene or health of the occupants or the condition of the building’.

Effective ventilation will remove excess water vapour from areas where it is produced in sufficient quantities in order to reduce the likelihood of creating conditions that support the germination and growth of mould, harmful bacteria, pathogens and allergies.

Building Standard 3.15 states:

‘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 the occupants as a result of moisture caused by surface or interstitial condensation’.

Condensation occurs in buildings when water vapour, usually produced by the occupants and their activities, condenses on exposed building surfaces (surface condensation) where it supports mould growth, or within building elements (interstitial condensation).

An Introduction to Property Flood Resilience of existing buildings

The owners or occupiers of existing buildings are not obliged to install PFR under the building standards system. This advice does not apply in most cases to warrantable work, however it is beneficial to those involved in the design and installation of PFR to existing properties which are at risk of or have been damaged by flooding.

In buildings that have been affected by a flood event, or are in a flood risk area, there is the opportunity to assess the risk of flooding and retrospectively install PFR measures. The installation of PFR measures can assist in protecting the property and occupants against future flooding events, limiting damage and reducing both remedial works and time spent out of the property.

PFR measures are most useful where a proactive approach is taken and measures are installed prior to rather than following a flood event. An opportunity to take this proactive approach is when property refurbishment work such as energy and insulation upgrades are being carried out, designers should consider, where flood risk exists, how PFR can be retrofitted to mitigate the impact of flooding.

The guidance in clause 3.3.3 is specific to new build work, however, it can be used when considering PFR for existing properties. Core documents such as BS 85500: 2025, BRE Good Building Guide 84 and in particular the CIRIA Code of Practice will provide designers with an invaluable source of information and PFR measures.

‘Flood Re is a reinsurance scheme which will allow insurance companies to offer more affordable flood cover to those living in high risk areas. As part of this scheme Build Back Better is available to participating insurers to offer homeowners additional funding to install PFR measures when repairing their properties after a flood event’.

The Scottish Flood Forum (SFF) is a charity whose aim is to reduce the impacts of flooding on individuals and communities, through providing immediate support and by establishing a network of community resilience groups in flood risk areas to equip communities to cope with the impacts and threat of flooding.

The SFF website provides information and resources on pre and post flood event advice and actions.

Good practice guide

Table 3 provides a suggested checklist for good practice on verifying building standards compliance on flooding and groundwater which can be used as part of the building warrant assessment process and as part of reasonable inquiry checks on site. It recommends checking on flood risk and communicating with the Local Authority flood officer, as appropriate, early on in the building warrant process. The checklist can also be reviewed to place any action already undertaken as part of a planning consent into account.

Good practice checklist

Building warrant

Is flooding and groundwater an issue at this site? (e.g. local knowledge and/or SEPA Flood Map check)

Building warrant

Has this issue been addressed during the planning application process? If not, consultation with the flood officer is recommended.

Building warrant

Are all relevant documents available? E.g.:

• Flood risk assessment
• Site report
• Design review (new build)
• Specified materials/building elements
• Existing Property Flood Resilience (PFR; if applicable)
• Proposed PFR solution (if applicable) and evidence of it following the Code of Practice as defined in CIRIA C790? (See accompanying table)
• And been accepted by the relevant officer (e.g. flood officer; structural engineer)?

Building warrant

If flood mitigation measures have been used do they meet the BSI Standards: BS 8000-0:2014, BS 8533:2017 and BS 85500: 2025

Inspection

Does the location of the development and finished floor level comply with the approved plans?

Inspection

If PFR has been used is there evidence of it following the Code of Practice as defined in CIRIA C790? (See accompanying table)

Submission of Completion Certificate by relevant person

Is evidence of the following available?

All flood risk:

• Does the location of the development and finished floor level, including basements comply with the approved plans?
• For PFR; required information:
• On-site checks/inspections to verify specified PFR solutions have been achieved (Construction Compliance and Notification Plan, CCNP).
• Evidence of in-situ and wet testing.
• For PFR; optional information:
• Receive post installation audit (PiA) if available.
• Receive Homeowner Flood Plan (or O&M manuals) if available.

With respect to PFR, the CIRIA C790 Code of Practice is recommended including Post Installation Audit of PFR measures. The alignment of CIRIA C790 with building standards processes is summarised in Table 4.

CIRIA C790 Alignment with Building Standards Summary

CIRIA C790 Standard

Stage 1 Hazard Assessment

Stage 2 Property Survey (existing buildings)

Stage 3 Options development and design

Building Standards process

Building Warrant:

Flooding type(s), depth, speed, duration and frequency.

Property survey (existing) and design review (new build); specified materials/building elements, and proposed PFR solution.

Relevant Reference

C790B: Chapter 12

C790B: Chapter 13 BS 8550: 2025 Clause 4.4

C790B: Chapters 14, 23, 25

CIRIA C790 Standard

Stage 4 Construction

Stage 5 Commissioning and handover

Stage 6 Operation and maintenance

Building Standards process

Completion Certificate:

On-site checks/inspections to verify specified PFR solutions have been achieved (CCNP¹).

Evidence of in-situ and wet testing.

Receive post installation audit (PiA).

Receive Homeowner Flood Plan (or O&M manuals).

Relevant Reference

C790B: Chapter 15

C790B: Chapter 16

C790B: Chapters 17 and 24

Notes:

1. Construction Compliance and Notification Plan (CCNP)

Question 9 – Do you agree that the introduction of the guidance in Annex 3.B offers further useful information to support the informed practice in flood risk assessment and the application of flood resilience principles?

Yes / No

Please provide any additional comments you may have.

Question 10 – Are there any other issues that you consider Annex 3.B could address to further improve knowledge and understanding of this topic?

Yes / No

If you answered ‘Yes’, please provide your comments