Domestic extensions

If you need more space and you do not want to move house you may wish to consider extending your home. Your home is probably your most valuable asset so it is important that your extension project is carefully planned. This guide is not a substitute for professional advice but has been written to provide you with useful information about how the building regulations will affect your extension.


Most houses are suitable for extending, providing that you have the space. When you are thinking about whether your house is suitable for extending you might like to consider:

  1. How will you access your extension?
  2. What effect will your extension have on the circulation in and around your home?
  3. What effect will your extension have on your existing house and garden? Will it block out light from existing rooms or make some rooms unusable? Will you still be able to get into your garden?
  4. Is your existing house built from an unusual construction for example prefabricated panels, concrete frame etc?

If you can resolve all of these issues then your house may well be suitable for extending.

Technical issues - how to proceed

Extensions are complex projects and unless you are experienced in construction you will need to get some professional advice. 


The foundations are one of the most important parts of your extension and often one of the most expensive. For domestic extensions trench fill foundations are the most common, these should be taken down into firm natural ground and should generally be a minimum of 1m deep. In areas with clay subsoil trees up to 20m away can have a significant effect on foundations which generally means that they need to be deeper, sometimes as deep as 2.5m. If you are concerned about any trees please get in touch with us and we will give you some advice regarding foundation depths.

Ground floor

The ground floor of your extension performs a number of tasks: it must support the floor loading, keep out damp and provide thermal insulation. Generally a ground floor is a multilayer structure, the top soil under the extension floor area is removed and a layer of compacted stone is placed over the site. This is blinded with sand and a layer of 1200g polythene is then placed over the sand and lapped in with the damp proof course in the wall. A layer of insulation is then provided and a concrete slab at least 100mm thick is poured over the insulation (some insulation may require an additional membrane). The concrete can either be float finished or a screed applied at a later date. On some sites where the ground floor is significantly higher than external ground level or where the site has been affected by trees, a suspended floor may be needed, these can be formed from either concrete or timber and if you need any guidance regarding suspended floors, please get in touch with us for advice.


The walls of your extension must carry the loads from the floors and roof, keep the weather out of the extension and provide thermal insulation. Cavity walls are commonly used for domestic extensions. These are made up from bricks and blocks and the cavity is filled with insulation as the work proceeds. When building walls remember to ensure that you have adequate buttressing at the corner of your extension, lintels over all openings, wall ties to join the leaves of your cavity wall together and a suitable damp proof course. If you are building up against your neighbour’s house you will also need to ensure that your wall provides adequate sound resistance.

First floor 

Two storey extensions will require a first floor, these are generally made up from timber floor joists which span between load bearing walls, they support floor boarding above and plasterboard is then fixed to the underside of the joists to provide a ceiling finish and fire resistance. The size of the floor joists will depend on the span so please contact us for advice on the joist size required. The floor will also need to include sound insulation and in domestic extensions, 100mm of sound deadening mineral wool placed between the joists is generally sufficient.

The roof structure

The roof of your extension will need to be designed to keep out the rain and snow and may need to cope with some light loft storage loading. Generally two types of roof are used for domestic extensions:

Flat roofs: This is the simplest type of roof structure and for some extensions,generally single storey, a flat roof can provide a practical and economic solution. Timber joists are used to span between the loadbearing walls and beams and these are covered with a plywood decking laid on firring strips to provide a fall. Thermal insulation is then placed over the roof and it is generally finished with a waterproof covering of three layers of bonded roofing felt. Critical things to consider in this type of roof are the size and support of the roof joists and the way that the roof will be insulated and, if necessary, ventilated.

Pitched roofs: If a flat roof is not suitable for your needs you are likely to require a pitched roof. These are generally more substantial structures that are finished with roof tiles or slates. The supporting structure of the roof can be formed in two ways:

  1. Trussed rafter roofs: these are quick to construct, measurements are taken from site and roof trusses are made up in a factory, they are then delivered to site ready for installation. Each roof is individually designed by the roof truss manufacturer using specialist computer software and the carpenter’s time on site can be significantly reduced.
  2. Traditional roofs: A carpenter cuts a traditional roof on site. The roof structure will generally be designed by an architect or structural engineer and the timber is then delivered to site where the carpenter will set out the roof and cut each of the individual timbers to size before installing them. This type of roof offers the greatest flexibility in roof shape and is often the only way of roofing complicated extensions especially where the new roof must join onto an existing structure. The size of the timbers and supporting beams required in a roof will depend on the loadings and spans involved in each case, complex roofs will require a structural engineer’s design but our Building Control Surveyors will be happy to assist your builder in designing simple roof structures. Once the support is in place the roof must be covered to provide weather protection, pitched roofs are generally finished with tiles or slates with a layer of roofing membrane or felt under them. The tiles are supported by the rafters via a series of timber battens. To provide adequate weather resistance the tiles overlap each other and they must have an adequate pitch. For extensions it is common to use tiles or slates that match the main house although this is not always possible if the extension roof has a very low pitch. Our Building Control Surveyors will be happy to assist you with any enquiries that you have about roof finishes.



If your extension has more than one storey you may need to install a staircase and careful design of this can be critical to the success of the extension. If a stair is installed it should be designed in accordance with the following guidance.

Width: there is no minimum width for stairs in the building regulations however they will need to be usable. Generally stairs are 850–1000mm wide.

Pitch: the maximum pitch for the stair should not exceed 42 degrees.

Rise and going: the maximum rise of each tread of a domestic stair should not exceed 220mm and the going should be at least 220mm.

Headroom: the clear headroom over the stair should be at least 2m.

Handrails and Balustrades: the stair should be provided with a handrail at least 900mm high and any exposed edges of stairs or landings should be provided with balustrading at least 900mm high.

Fire precautions

It is important that you consider fire precautions when you are designing your extension. The most dangerous fires generally occur at night when everyone is asleep and to give you awareness of a fire the building regulations suggest that mains operated smoke detectors should be installed on each floor of the house when it is extended. To prevent people being trapped by a fire all rooms that do not open directly onto a hall and all first floor rooms should have a window or door that is large enough for people to escape through. If you are using a window as your secondary fire escape it should have a clear opening of at least 0.33m2 at least 450mm wide with a sill height of between 800 and 1100mm above floor level. Fire is a very dangerous thing and careful design and planning are required to ensure that the risks it poses are minimised.


Fresh air is essential to healthy living and the building regulations require your extension to have adequate ventilation, generally an opening window with a ‘trickle vent’ is all that is required, the window should have an openable area equivalent to at least 1/20th of the floor area of the room that it is ventilating. The trickle vent can be a small slot type vent that you can leave open to allow some background ventilation without the need to open the window, generally these are found in the top of the window frame. If your extension contains a kitchen, utility room or bathroom you will need to provide an extract fan in these areas and your Building Control Surveyor will be pleased to provide you with more detailed advice when they call on site.


It is usually possible to connect drainage from extensions into the existing drainage systems. Drainage can be divided into two types, foul water and rainwater and generally speaking the drainage systems should be kept separate. Foul drainage is generally discharged through a series of pipes and manholes to a public sewer although some properties will have septic tanks or private sewage treatment plants. When planning your extension look for manholes and try and find out where your drains are running so that you can work out how any new drains will connect to them. It is important that all new underground drain pipes have a diameter of at least 100mm so that they do not block or freeze, are watertight and have manholes or access points so that any blockages can be cleared. Where possible rainwater drainage should not be discharged to foul sewers as this can cause problems with flooding, the preferred solutions are to discharge rainwater to soakaways located in your garden at least 5m from any building if ground conditions permit or to storm water sewers if not.


Most extensions will need to be heated and you will need to check with your heating engineer that your existing system has sufficient capacity to heat your extended house. You may also need to move your boiler, if for example, your extension will cover the flue outlet. Any alterations to your heating system should be carried out by a suitably qualified plumber or heating engineer registered with Gas Safe for gas fired boilers or OFTEC for oil fired boilers. Any new boilers will need to be highly efficient condensing boilers and the new radiators that you install in your extension should be fitted with thermostatic radiator valves so that you can ensure that they use heat efficiently.

Sound insulation

To reduce unwanted noise the walls and floor around bedrooms will need to be insulated to reduce sound transmission, this is generally achieved by placing 100mm of sound deadening quilt in the floor void and in the partitions around the bedrooms.

Electrical installations

As part of the building regulations process you will need to supply British Standard Test Certificates for most new electrical installations, when selecting your electrical contractor please ensure that they are competent to provide you with these test certificates as otherwise you are likely to incur additional costs for testing the circuits.


To reduce the risk of people injuring themselves, glazing in and around doors and all glazing within 800mm of floor level should be either toughened or laminated glass.