Building an extension
Many people still make the choice to build an extension without fully understanding the legal, planning and construction issues you will face along the way. So before you start, it pays to have a good idea of what actually goes into building an extension. One of the first questions you are likely to ask yourself is if an extension will add value to your home. Local estate agents are probably best placed to give you an estimate on how much value might be added when the extension is complete. Your home has to adapt to your changing needs over time, so if you require more room to accommodate a growing family or an elderly relative, building an extension could provide a viable alternative to the stress and upheaval of moving home. Costs will obviously depend on the extent of your project and whether it’s a single or two-storey extension. Construction rates are normally calculated per square metre but prices can also differ significantly. As a rough guide, expect to pay anything from £1,300-£2,100 per square metre for a single storey extension and between £1,500- £2,500 per square metre for a two-storey extension.
Other cost factors to consider will be the quality of the materials and any new fixtures and fittings the type of construction methods chosen and any local constraints that maybe applied due to site circumstances these may all have an impact upon the overall cost of the schemes. (e.g. sub soil conditions proximity of trees depth of drains planning constraints.
Extensions
Building Regulations
Although planning permission may not be required for smaller extensions, building regulations approval is required for all extensions, no matter how small they might be even if an extension is being re-built Building regulations will apply to structure, fire safety, energy efficiency, insulation, ventilation and materials – so it’s important to be aware of the rules.
Exception conservatories / porches see separate guidance within the chatbot.
Issues that will be considered and addressed as part of the Building Regulation process for an extension;
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, and if there are any trees, drains and/or sewers, and any adjoining. Some foundations may need to be designed due to trees, poor ground conditions or where there are high or unusual loadings.
Floor
A floor will need to provide structural support for the room’s contents and users as well as resistance to ground moisture (damp) and heat loss (thermal insulation).
There are generally two types of floor construction used in extensions: solid and suspended. Solid floors require the ground to be made up in layers and would normally include a sub base of compacted hard core, sand, damp proof membrane, insulation and concrete. Suspended floors are normally made up of two materials, either timber joists or a concrete beam system which are then covered with either floor boards or high quality sheets of tongue and groove. There are a number of variations on these types of floor, mainly depending on the intended use of floor area and the required floor finish. If you need more guidance regarding the type of floor you will need, please contact your local authority building control team.
Walls
The walls of the extension will carry the loads from the floor and roof and 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 which are filled with a suitable insulating material as works progress. You can also use a solid wall if in blockwork this will need to be weatherproofed normally by rendering and the wall will also the provision of additional thermal insulation being applied internally or externally.
The following should also be provided:
- Adequate buttressing to the existing property
- Lintels over all openings
- Wall ties to join cavity walls together
- A suitable damp course
- If building up against neighbour’s house, adequate sound resistance is provided.
Fire precaution
One of the main pillars of the building regulations is to ensure people have the means to escape in the event of fire. Rooms that do not open directly onto a hall, and all first floor habitable rooms in a two-storey extension, should have a window or door that is large enough for people to escape through in the event of a fire. Windows must have a clear opening of at least 0.33m2 and measure at least 450mmx450mm. It is a requirement that mains-operated smoke detection systems are installed throughout and interlinked with a battery-operated backup system.
Drainage issues
The key issue regarding drainage is the connections to existing drains and water supply. Drainage is divided into two main types – foul water and rain water – which have to be kept separate. Foul drainage normally runs to a public sewer through underground pipes which are accessed via manholes. You may be to either a separate independent drainage system or to a soakaway. You are therefore advised to check with your local authority building control team and the water utility company regarding what type of surface water drainage is provided to your property 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. If it is being built over, or even close to, a shared sewer, the work will need an agreement document and this can be costly, especially if it is felt that the sewer needs to be moved or a manhole is needed. may be to either a separate independent drainage system or to a soakaway. You are therefore advised to check with your local authority building control team and the water utility company regarding what type of surface water drainage is provided to your property and how any new surface water should be drained.
Electrics
Homeowners will need to demonstrate that all electrical work is compliant with Part P
(Electrical Safety) of the Building Regulations. This can be done through self-certification by an installer who is a member of one of the electrical competent person self-certification schemes. When the work is finished you should receive a compliance certificate that confirms the work complies with the building regulations.
Heating
To maximise the usability of the room you will probably want to install heating in your extension. The most effective way of doing this is to extend the existing central heating system but you will need to check with your plumber or heating engineer to ensure that your existing boiler has sufficient capacity to serve any additional radiators.
Windows and doors
All new windows must be fitted with energy efficient double glazed units. Building regulations also stipulate glazing in and around doors and all glazing up to 800mm (up to 1500mm in doors and adjoining sidelights) from floor level should be either toughened or safety glass to stop people injuring themselves.
Ventilation
All new rooms in an extension will need adequate ventilation for general health reasons and the type of room and its volume will determine how much ventilation is required. There are two general rules for ventilating a room: purge – this is achieved by opening the window; and whole building – this is 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 approaches to ventilation may also be acceptable, subject to agreement 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 a means of 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 will 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
When designing your extension the roof type, its insulation and support, and the headroom will all need to be considered. Designed to primarily keep out the elements, the roof will have to support the roof fabric and possibly renewable technologies such as solar panels. Generally, there are two 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 where simple will normally be designed by using tables as supplied by TRADA but 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 two forms: Trussed rafter roofs: This type of roof provides the structural framework to support the roof fabric.
Traditional pitched roof:
This type of roof structure where simple will normally be designed by using tables as supplied by TRADA but 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 simply supported beams over shorter spans 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.
Although not a comprehensive list of items considered this gives an indication of the sort of things you need to consider when building an extension.
