NHBC Standards 2011

8.1 Internal services


This Chapter gives guidance on meeting the Technical Requirements and recommendations for internal services, including plumbing, hot and cold water supply, and gas, electric and solid fuel heating installations.


8.1 - D1 Design standards
8.1 - D2 Statutory requirements
8.1 - D3 Structural stability
8.1 - D4 Precautions against ground hazards
8.1 - D5 Precautions against chemical attack
8.1 - D6 Water supply
8.1 - D7 Cold water service
8.1 - D8-D9 Hot water service
8.1 - D10-D11 Electrical service
8.1 - D12-D13 Gas service
8.1 - D14-D15 Space heating
8.1 - D16-D17 Soil and waste systems
8.1 - D18-D19 Provision of information

8.1 - D1
Design shall meet the Technical Requirements

Design that follows the guidance below will be acceptable for internal services.


8.1 - D2
Design shall comply with all relevant statutory requirements

Designs should be in accordance with relevant Building Regulations and other statutory requirements.


8.1 - D3
The design of internal services shall not adversely affect the stability of the dwelling

Notching, drilling and chasing to accommodate service pipes and cables should either:

  • comply with Sitework clauses 8.1 - S2(d) and (e), or
  • be designed by an Engineer.


8.1 - D4
Design of service entries shall take account of ground hazards

In certain parts of the country, special precautions are necessary to reduce the entry of radon gas. Areas in England and Wales where special precautions are necessary are detailed in BRE Report 211 'Radon: guidance on protective measures for new dwellings'.

Where landfill and other gases are present precautions are necessary to prevent entry. Details are shown in BRE Report 212 'Construction of new buildings on gas-contaminated land'.


8.1 - D5
Adequate precautions shall be taken to ensure services are not subject to chemical attack


Pipes and fittings for water services should be of materials which are resistant to corrosion. The recommendations of the water supplier should be followed as to the compatibility of the water supply with materials and fittings.


PVC covered cables should not be in contact with polystyrene insulation.


8.1 - D6
Water service design shall be in accordance with statutory requirements and based on the pressures and flow rates supplied from the incoming main

Water services should be in accordance with relevant Building Regulations and other statutory requirements.

The design of the water service should be based on a minimum dynamic pressure of 1.5 bar at the stopvalve inside the home.

The design should ensure that a minimum flow rate of 20L/min is available at the stopvalve inside the home.

The design should take account of:

  • pressure and flow rate reductions where there are long distances between the water main and the stopvalve inside the building. This may involve increasing the diameter of the supply pipe
  • pressure fluctuations, or surges, which may occur within the system and the potential damage they may have on fittings. Surge arresters may need to be installed at suitable points within the system.


8.1 - D7
Adequate cold water services shall be provided

Items to be taken into account include:

(a) drinking water

Drinking water should be provided at the kitchen sink. The supply should come direct from the supply pipe or, where this is impracticable, from a storage cistern containing an adequate supply of wholesome water. Further information is given in Water Regulations and guides.

(b) cold water storage

Cold water storage should be provided:

  • to supply an open vented hot water storage system
  • where required by the water supplier
  • to supply those cold water outlets not connected to the supply pipe.

The cold water storage capacity should take account of the guidance in BS 6700, which recommends:
"In small houses it is usual for storage cisterns supplying only cold water fittings to have a capacity of 100L to 150L, and double this capacity if supplying all water outlets, hot and cold.

In larger houses a total storage capacity of 100L per bedroom is recommended".

Primary feed cisterns for indirect water heating systems should be of adequate capacity.

Cold water storage cisterns should be accessible for inspection and maintenance.

Storage cisterns should have rigid close fitting covers (which are not airtight) and which exclude light and insects.

(c) warning and overflow pipes

Warning and overflow pipes should be of adequate size and be provided from all cold water cisterns to a suitable discharge outside of the building. Where permitted by water regulations, the discharge may be internal provided it is conspicuous.

(d) reducing the risk of freezing

Pipes and cisterns should be located in the warm envelope of the home to reduce the risk of freezing.

Where pipes and cisterns are placed in unheated spaces, they should be adequately insulated to reduce the risk of freezing (see BS 6700 and Appendix 8.1-A).


8.1 - D8
An adequate hot water service shall be provided in accordance with statutory requirements

Hot water services should be in accordance with relevant Building Regulations and other statutory requirements.

8.1 - D9
The hot water services shall be adequate for the likely demand and consumption

Items to be taken into account include:
(a) types of system

  • instantaneous systems (combination boiler)
    These systems produce hot water on demand, but generally at significantly lower flow rates than storage systems.  They should only be used where the simultaneous demand for hot water is limited e.g. in homes with only one bathroom or shower room, unless the boiler manufacturer can show that the boiler is capable of producing hot water simultaneously to outlets in accordance with clause D9(c).
    For storage combination boilers, whose characteristics are similar to storage systems, the storage capacity should meet with the guidance in (b) below.
  • storage systems (vented cylinder, unvented hot water storage system, primary store)
    These systems provide higher flow rates than instantaneous systems but require a suitable space for the siting of the storage vessel.

(b) hot water storage

Hot water storage for notional draw-off requirements should be not less than shown in the following table. Where appliances are to be installed requiring greater amounts of hot water, the storage capacity should be increased accordingly:

Home with
Shower only Bath only Bath and shower(s*) Two baths
60L 120L 145L 180L

*max. 2 showers (excludes instantaneous electric showers)

For systems heated by off-peak electricity, the storage capacity should be in accordance with the recommendations of the electricity supplier.

Where boiler outputs are controlled and can prioritise hot water then storage capacities can be less than the figures in the table. Some storage combination boilers and combined primary storage units (CPSU) have this facility. The manufacturer should be consulted on the appropriate storage capacity for the likely hot water demand.

For homes with a single bathroom/shower room, it is assumed that immediately after filling a bath, some hot water may be required for kitchen or wash basin use, but a second bath will not be required for another 20 or 30 minutes.

For homes with two or more bathrooms, it is assumed that all the installed baths will be filled in succession and that some hot water may be required for kitchen or wash basin use immediately after.

The quantities of hot water stored are based on a water draw off temperature of 60ºC.

(c) flow rates and temperature

The system should be designed and installed so that the following flow rates and temperatures are available:

Outlet Flow Rate L/s Supply temperature

Design Rate
(see note 1)
Minimum Rate
(see note 2)

Bath (from storage) 0.3 0.2 60ºC
Bath (from combi) 0.2 0.15 40ºC
Shower (see note 3) 0.2 0.1 40ºC
Wash Basin 0.15 0.1 40ºC
Sink 0.2 0.1 60ºC


  1. The design flow rate should be available at each outlet when the total demand does not exceed 0.3L/s. When simultaneous discharge occurs, the flow rate at an individual outlet should not be less than the minimum rate.
  2. The minimum flow rate should normally be available, based on clause D6, but may be less if the pressure and flow rate of the incoming supply falls below 1.5 bar.
  3. Excludes instantaneous electric showers.
  4. Further information on flow rates and temperatures can be found in BS 6700.

(d) unvented hot water storage systems

Unvented hot water storage systems should be assessed in accordance with Technical Requirement R3 or meet the requirements of BS 7206, and be the subject of a third party certification scheme (e.g. Kitemarking).

(e) safety in showers

Where a shower is installed, adequate provision should be made to ensure that the outlet temperature of the water is not seriously affected by the use of other hot or cold outlets in the dwelling. This may be achieved by the provision of a thermostatic shower mixing valve, appropriate design of pipe sizes or dedicated supplies.


8.1 - D10
The electrical installation shall be provided in accordance with relevant regulations

All electrical installations should comply with BS 7671.

Cables which are covered or surrounded by thermal insulation may need to be de-rated. Guidance is available in the BRE report 'Thermal insulation: avoiding risks'.

8.1 - D11
An adequate electrical service shall be provided

Items to take into account include:

(a) cooking
In all dwellings, a minimum 30A electricity supply, suitably switched and terminated, should be provided to the cooker space.

If a cooker panel is provided it should be located to the side of the cooker space. Where a gas supply is provided to the dwelling, a 13A socket outlet should be positioned at the cooker space.

(b) socket outlets

Rooms should be provided with not less than the following 13A outlets. Dual outlets count as two.

Room Outlets Notes
Kitchen/utility 8 Where homes have separate areas, the kitchen should have a minimum of 4 outlets, and the utility room 4.
Where appliances are provided, at least 3 outlets should be free for general use
Dining room 4  
Living or family room 8 At least 2 outlets should be near the TV aerial outlet


6 (4) 6 for main bedroom
4 for other bedrooms
Landing 2
Hall 2


(c) lighting

Every room should have at least one fixed lighting outlet.

Lighting outlets should be provided to halls, landings and staircases. At each floor level two-way switching should be provided to staircases.

In common areas to dwellings, artificial lighting should be provided, controlled by either manual switching by persons using the area or automatic light sensitive controls.

(d) fixed appliances


Extract fan systems should be designed in accordance with the manufacturer’s recommendations. Ducts passing through unheated spaces, such as a roof void, to the outside air should be insulated to prevent condensation affecting the operation of the fan. Alternatively, the ducting should have a means of collecting the condensate and draining it to the outside. Where ducting is part of a mechanical ventilation and heat recovery system it should be insulated in accordance with the manufacturer’s recommendations.


Where a gas appliance requires an electrical supply, a suitable fixed spur or socket outlet should be provided.

(e) television

A concealed co-axial cable should be provided from the roof void to a terminal outlet in the main living room. Where the co-axial cable is not provided, a conduit and draw wire or an alternative should be provided. The provision of an aerial is not required.


8.1 - D12
Where provided, gas services shall be adequate and in accordance with relevant standards and codes

All gas services must comply with the Gas Safety (installation and use) Regulations.

British Standards relevant to the design of gas installations include BS 6891 and for Butane and Propane gas, BS 5482. Other authoritative publications such as those prepared by the Institution of Gas Engineers and Managers and Gas Safe Register (GSR) can be used.

Gas pipework to be installed in timber frame construction should allow for the likely differential movement. Reference should be made to Chapter 6.2 'External timber framed walls'.

Where a gas supply is provided, there should be a gas point at the cooker space.

For details of hearths, flues and air supply, refer to Chapter 6.8 'Fireplaces, chimneys and flues'.

8.1 - D13
Where provided, gas services shall be adequate

Meters and associated equipment should be located where they are reasonably accessible and not subject to damage. Domestic meters may be of the following type:

  • built-in – to the outer leaf of the wall
  • surface-mounted – on an external wall
  • semi-concealed – sunk into the ground adjacent to the outer wall
  • individually purpose-made compartments – in accordance with the recommendations of BS 6400.


8.1 - D14
Where space heating is provided it shall be in accordance with relevant standards

British Standards relevant to heating systems include BS 5449, BS 5410 and BS 8303.

Underfloor heating systems should be designed in accordance with BSRIA guides AG12 and 13.

8.1 - D15
Space heating shall be adequate

Items to be taken into account include:

(a) minimum standards for living room heating

The main living room of a dwelling should have a heating appliance or a heat output as part of whole home heating which is capable of maintaining a temperature of at least 21ºC in the room when the outside temperature is -3ºC.

(b) minimum standards for whole home heating

The provision of whole home or central heating is discretionary. Where it is provided, it should be designed to recognised standards and based generally on the following:

  • external temperature -3ºC
  • the design temperatures and ventilation rates given in the table below:
Room Room temperature
Ventilation rate
(air changes per hour)
Living room 21 1.5
Dining room 21 1.5
Bedroom 18 1
Hall and landing 18 1.5
Kitchen 18 2
Bathroom 22 2
Toilet 18 2


  1. The number of air changes per hour from kitchens and bathrooms should take account of any mechanical ventilation installed.
  2. Where rooms contain open flued appliances, the rate of air change used for the design should be increased (see BS EN 12828).
  3. In case of dispute, the design temperatures adopted should be verified by calculations and not by performance tests.

(c) safe operation of heating appliances

Reference should be made to Sitework clause 8.1 - S7 and Chapter 6.8 ‘Fireplaces, chimneys and flues’ for guidance on:

  • location of appliances
  • provision for supply of combustion air and removal of combustion products
  • separation from combustible materials.


8.1 - D16
Internal soil and waste systems shall be designed in accordance with relevant statutory requirements

Designs should be in accordance with relevant Building Regulations and other statutory requirements.

8.1 - D17
Internal soil and waste systems shall be adequate

Items to be taken into account include:

(a) disposal of effluent from the building

Soil and waste systems should comply with any specific requirements from the water supplier.

Guidance and recommendations for building drainage and sanitation are given in BS EN 752 and BS EN 12056.

(b) entry of foul air from the drainage system to the building

Soil and waste systems should be arranged so that:

  • each branch is adequately ventilated
  • foul air from the drainage system cannot enter dwellings.

Ventilation should be provided at the head of underground drains. This may be by a soil pipe or separate ventilation pipe.

Where a soil pipe or ventilation pipe is less than 3m away from an opening into the building, it should extend at least 900mm above that opening.

Where a ventilation pipe is less than 3m away from an opening into the building, it should extend at least 900mm above that opening

(c) air admittance valves

Air admittance valves only allow air to enter the drainage system. Their use does not avoid the need to adequately ventilate the drainage system.

Where air admittance valves are used to terminate soil pipes, they should comply with BS EN 12380 or be assessed in accordance with Technical Requirement R3. Valves within the building should be:

  • positioned in areas which are not liable to freezing
  • positioned in areas which have adequate ventilation
  • accessible for maintenance.

(d) entry of vermin

Entry of vermin should be prevented.

(e) noise transmission

Precautions should be taken to limit noise transmission from rooms containing WCs, for example:

  • soil pipes passing through dwellings should be encased and insulated; the insulation should be continued through the thickness of any sound-insulating floor
  • walls between living rooms and rooms containing WCs should be insulated as recommended in Chapter 6.3 ‘Internal walls’ (Design). In England and Wales, reference should be made to statutory requirements.

Sound insulation should be detailed in accordance with Sitework clause 8.1 – S8(c).


8.1 - D18
Designs and specifications shall be produced in a clearly understandable format and include all relevant information

For internal services drawings should show:

  • location of sanitary fittings
  • drainage runs
  • location and size of cold water storage cisterns
  • location and size of hot water storage cylinder
  • hot and cold water pipe runs
  • heating boiler and heat emitters
  • central heating pipe runs
  • gas supply pipe runs
  • electrical outlets, switches and consumer unit.
8.1 - D19
All relevant information shall be distributed to appropriate personnel

Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.