Building an extension

Building an extension can involve legal, planning and construction issues. Before you start, it pays to have a good idea of what goes into building an extension. 

This page gives an idea of some of things to consider if you’re thinking about building an extension.

Costs of an extension

Costs will depend on the extent of your project and whether it’s a single or 2-storey extension. Construction rates are normally calculated per square metre, but prices can differ significantly. As a rough guide, expect to pay anything from:

  • £1,300-£2,100 per square metre for a single storey extension 
  • £1,500- £2,500 per square metre for a 2-storey extension

Other factors that will affect cost are:

  • the quality of the materials 
  • any new fixtures and fittings 
  • the type of construction methods chosen 
  • any local constraints that maybe applied due to site circumstances 

Building Regulations

Building regulations approval is required for all extensions, no matter how small they might be. They also apply if you are re-building an extension. 

Building regulations will apply to: 

  • structure
  • fire safety
  • energy efficiency
  • insulation
  • ventilation and materials

Conservatories and porches have separate guidance

As well as building regulations, you may need planning permission if it is not a smaller extension

Building regulations will consider these areas: 

Foundations

Foundations have to transmit the load or weight of the extension safely to the ground. 

The depth of the foundation will depend on a number of factors including: 

  • location and ground conditions
  • if there are any trees
  • drains and/or sewers 

Foundations may need to be designed if there are trees, poor ground conditions, or if they have to carry high or unusual loadings.

Floor

A floor needs to provide structural support for the room’s contents and users. It must also be resistant to ground moisture (damp) and heat loss (thermal insulation).

There are generally 2 types of floor construction used in extensions - solid and suspended:

  • solid floors require the ground to be made up in layers. They normally include a sub-base of compacted hard core, sand, damp proof membrane, insulation and concrete 
  • suspended floors are normally made up of 2 materials: either timber joists or a concrete beam system covered with either floor boards or high quality sheets of tongue and groove 

There are many variations on these types of floor. The type of floor you choose will depend on what you'll use the floor for and the finish you need. 

If you need guidance about the type of floor you'll need, contact your local authority building control team. 

Walls

The walls of the extension will carry the loads from the floor and roof. They must keep the weather out as well as provide thermal insulation. 

The most common type of wall construction for an extension is a cavity wall. Cavity walls are normally constructed from brick and blocks and filled with insulating material. 

You can also use a solid wall if in blockwork. This will need to be weatherproofed normally by rendering, and will need additional thermal insulation applied internally or externally.

You'll also need to have:

  • adequate buttressing to the existing property
  • lintels over all openings
  • wall ties to join cavity walls together
  • a suitable damp course
  • adequate sound resistance, if the building is against neighbour’s house 

Fire precaution

Building regulations make sure people have the means to escape in a fire.

You must have a window or door large enough for people to escape through for:

  • rooms that do not open directly onto a hall 
  • all first floor habitable rooms in a 2-storey extension 

Windows must:

  • have a clear opening of at least 0.33m2 
  • measure at least 450mmx450mm

Mains-operated smoke detection systems must be installed throughout and interlinked with a battery-operated backup system. 

Drainage issues

Drainage issues generally relate to connections to existing drains and water supply. 

There are 2 main types of drainage: 

  • foul water 
  • rain water

These have to be kept separate. 

Foul water normally runs to a public sewer through underground pipes accessed by manholes. You may need to link to either a separate independent drainage system or to a soakaway. 

Check with your local authority building control team and water utility company to find what type of surface water drainage your property has and how any new surface water should be drained. 

Water utility companies will need to be contacted if your extension is being built over a sewer or near to their asset for which they have a responsibility. 

You'll need an agreement document if your extension is being built over, or close to, a shared sewer. This can be expensive, especially if a sewer needs to be moved or a manhole is needed. 

Electrics

All electrical work must comply with Part P (Electrical Safety) of the Building Regulations. An installer who is a member of one of the electrical competent person self-certification schemes can self-certify this. 

When the work is finished, you should receive a compliance certificate that confirms the work complies with the building regulations.

Heating

You may want to install heating in your extension. If you want to do this by extending existing central heating, check with your plumber or heating engineer that your existing boiler has capacity for additional radiators. 

Windows and doors

All new windows must be fitted with energy efficient double-glazed units. Building regulations also stipulate glazing should be either toughened, or safety glass used, for glazing: 

  • in and around doors 
  • up to 800mm (up to 1500mm in doors and adjoining sidelights) from floor level

This is to stop people injuring themselves. 

Ventilation

All new rooms in an extension will need adequate ventilation. The type of room and its volume determine how much ventilation is required. 

There are 2 general rules for ventilating a room: 

  • purge – achieved by opening the window
  • whole building (also known as ‘trickle’ ventilation) which is incorporated into window framework, or by some other means

Both of these forms of ventilation are normally required. However, other ventilation approaches may be acceptable if they are agreed with your local authority building control team. 

Any mechanical system you wish to incorporate will need to meet the requirements specified in the building regulations. 

Any new kitchen, bathroom (or shower room), utility room or toilet should be provided with extract ventilation to reduce condensation and remove smells. 

The minimum requirement to comply with building regulations is an opening window with a ‘trickle’ vent equivalent to at least 1/20th of the floor area of the room. 

Extractor fans need to be fitted in any new bath or shower rooms that do not have opening windows. These should be triggered by the light switch with overrun timers which allow the fan to remain on after the light is turned off. 

Roofs

Roof type, insulation and support, and headroom will all need to be considered. The roof will have to support the roof fabric and possibly renewable technologies (such as solar panels). Generally, there are 2 types of roof suitable for extensions – flat and pitched. 

Flat roofs

Flat roofs can provide a practical and economical solution for homeowners. The key elements from a building regulations perspective will be the size and support of the roof joists. This type of roof structure will normally be designed using tables as supplied by TRADA. More complicated designs may be designed by an architect or structural engineer as the size of the timbers and supporting beams will depend on specific loadings and spans. 

Pitched roofs

Pitched roofs are significantly larger structures than a flat roof and come in 2 forms: 

  • trussed rafter roofs - provide the structural framework to support the roof fabric 
  • traditional pitched roof - normally designed using tables as supplied by TRADA. More complicated designs may be designed by an architect or structural engineer as the size of the timbers and supporting beams will depend on specific loadings and spans

Structural steelwork and bearings 

For simple supported beams over shorter spans, the beam size and bearings can be agreed with your Local Authority Building Control Officer but more complicated designs will need to be designed by an architect or structural engineer as the size of the beams and supporting structure will depend on specific loadings and spans.