Building standards technical handbook 2020: domestic

The building standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004. This handbook applies to a building warrant submitted on or after 1 March 2021 and to building work which does not require a warrant commenced from that date.

3.27 Water efficiency

Mandatory Standard

Standard 3.27

Every building must be designed and constructed in such a way that sanitary facilities with water efficient fittings which are designed for the prevention of undue consumption of water are installed.


This standard applies only to a dwelling.

3.27.0 Introduction

Using less water saves energy and reduces Carbon Dioxide (CO2) emissions. This is achieved by reducing the energy that is used in key areas including:

  • treating water to a standard suitable for drinking

  • distribution of water to homes

  • collection and pumping of generated wastewater

  • treatment of the wastewater generated

  • heating of water for health and hygiene  

Scottish Water estimate that the average consumption of wholesome water per person in Scotland is around 150 litres per day.  Water consumption has increased over the last few decades and is projected to continue to rise.    Around 30% of the average household’s heating bills are spent on heating water for sanitary, health or hygiene purposes. Therefore the provision of sanitary appliances and fittings that use water more efficiently can assist in the reduction of associated carbon emissions and the home owner’s energy costs.  

The Scottish Water Byelaws set requirements which must be adhered to in all properties that have a public water supply.  They cover the design, installation and maintenance of plumbing systems, water fittings and water-using appliances. Scottish Water now require all new industrial or commercial buildings to be metered. Meters may also be installed in dwellings if requested.  

Further information on water efficiency and related carbon emission savings can be found on the websites of Scottish Water,  Waterwise and the Energy Saving Trust.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.27.1 Water use

Water is used for various purposes within a dwellings.  The national independent organisation ‘Waterwise’ advise that the daily percentage of demand in key use areas of the total supplied water for dwellings to be as follows:

WC Flushing Baths and taps Laundry Showers Drinking and other Washing up External
30% 21% 13% 12% 9% 8% 7%

The actual water used within a dwellings will be influenced by both the behaviour of the occupants in how they use the sanitary facilities and associated water fittings and also the volume of water discharged from them. Therefore, controlling the volume of water discharged from sanitary facilities and fittings should contribute to reducing the use of water.  

Efficiency measures can be applied to most sanitary facilities with varying degrees of benefits. For certain sanitary facilities reducing the water use will be related to its function. For example a bath is generally filled to a water level that suits the occupant and therefore controlling water flow from taps to that facility would only serve to extend the amount of time taken to fill the bath.  

Most WCs produced today are now of the dual flush type which give users the option of a reduced flush, generally for liquid waste or a full flush for solid waste. The flush volume will generally be achieved through the valve mechanism of the WC cistern.  

To reduce the water flow rates at taps for wash or hand rinse basins (WHBs) options include the installation of flow restrictors, or aerators. These may be fitted in either the water supply pipes serving the sanitary facilities or incorporated within the tap components.

3.27.2 Water efficient fittings

Water efficient fittings should be provided to all WCs and WHBs within a dwelling.  

Dual flush WC cisterns should have an average flush volume of not more than 4.5 litres. Single flush WC cisterns should have a flush volume of not more than 4.5 litres.  

Taps serving wash or hand rinse basins should have a flow rate of not more than 6 litres per minute.  

Many bathroom and fittings manufacturers are now using the latest technology to offer comprehensive portfolios of water efficient products that deliver satisfactory performance. Further guidance and comparison of water efficient appliances and fittings for the industry can be obtained from schemes such as the water efficient product labelling scheme from the Bathroom Manufacturers Association.  

When specifying water efficient fittings consideration should be given to the operational flow rates that some heating or hot water appliances, such as combination boilers, need to activate their water heating function.

When installing low volume flush WCs, the pipe diameter, discharge and gradient inter-relationship of the drainage system is critical in order that the new and any existing sections of the drain operate as intended.  

Plumbing and associated water installations should be carried out and commissioned by persons who possess sufficient technical knowledge, relevant practical skills and experience for the nature of the work undertaken.  

An approved Certifier of Construction, who has been assessed to have the professional skills and relevant experience, can certify compliance of plumbing, heating or drainage installations.

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