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Last edited 16 May 2018
Building an extension
For a detailed description of the tasks necessary to undertake a self-build project see Self-build home: project plan.
Building an extension to a house is an increasingly popular choice for homeowners seeking to maximize the potential from their property space. With fees and stamp duty to consider as well as the upheaval entailed with buying a new house and continually increasing property prices, an extension can be seen as a more economical option.
 Types of extension
A garage is a common type of single-storey extension that is constructed to adjoin an existing building, and can be useful for keeping a vehicle inside, storage space, and so on.
For more information, see How to build a garage.
This is a single-storey attachment to the front of a house. Due to their relatively small size they may not require planning permission (although they may in a conservation area or for a listed building). Building regulations approval may be required if the porch is not separated from the house by an internal door and is heated, or if there are structural, accessibility or drainage implications.
For more information, see How to build a porch.
These are simple single-storey structures, usually made of glazing and a UPVC frame, although they can also be timber or aluminium. They may have a low-level brick wall around their perimeter upon which the framework site. The type and size of the conservatory will determine whether planning permission or building regulations approval is required or not.
Also known as a sunroom, an orangery is similar to a conservatory but has a solid roof and walls as well as glazing. This tends to make them more expensive than a conservatory, but sturdier and better at retaining heat. Similar planning permission and building regulations rules apply as for conservatories.
 Single-storey extension
A single-storey extension is built adjoining the existing house. The method of connection requires careful consideration, in particular, openings between the extension and the existing building, junctions with the roof structure, the positions of flues and drains, and so on. Planning permission may not be required in all instances, but building regulations approval will be.
 Two-storey/multi-storey extension
The same considerations will be relevant as for a single-storey extension, although the junctions and structure are likely to be more complicated and planning permission is very likely to be required.
This is where an extension is built over the top of an existing structure, most commonly a garage. In terms of ensuring stability, new foundations and inner leaf wall construction may be required since most garages have only single-skin brickwork. Since the quality of foundations cannot be guaranteed, suitable examination must be undertaken to ascertain the extent to which additional work needs to be done.
A basement is part of a building that is either partially or completely below ground level.
In cities such as London, due to the demand for housing, the price of land and the cost of moving, basements are being constructed or converted for living space. This is causing concern in some areas where very large, multi-storey basements are begin constructed, which can cause significant disruption to neighbours over a long period. As a result, some planning restrictions are being introduced. See Basements in buildings for more information.
Building regulations are applicable to the construction of new basements, and will cover areas including ventilation, drainage, ceiling heights, damp proofing, electrical wiring, water supplies, means of escape, and so on.
Loft conversions can increase the value of a home by up to 20%, and can add up to 30% more living space to a house.
In many instances, loft conversions can be completed as Permitted Developments, which do not require planning permission. However, if the plans fall outside of these limitations then planning permission will be required. Planning permission may also be necessary if the house is in a conservation area or a listed building.
Planning permission may not be required for extensions or house additions as they are considered to be permitted developments. However, some of the following limits and conditions apply to the extension are:
- Extensions must not exceed 50% of the total area of land around the 'original house' (the house as it was first built or as it was on 1 July 1948).
- The eaves and ridge height can be no higher than the existing house.
- Single-storey rear extension cannot be higher than 4 m.
- Multi-storey rear extensions cannot exceed the rear wall by more than 3 m in length.
- Single-storey rear extension cannot exceed the rear wall by more than 3 m in length (if an attached house) or 4 m (if a detached house).
- Two-storey extensions can be no closer than 7 m to rear site boundary.
- The roof pitch of extensions higher than one storey must match that of the existing house, as far as is practicable.
- Materials used must be similar in appearance to the existing building (not relevant for conservatories).
- Extensions forward of the front or side elevation of a house facing a highway are not permitted.
- Balconies or verandas are not permitted.
Where these requirements are not met, planning permission will be required. However, there are local variations, and Article 4 directions can be used locally to remove permitted development rights, so it is always worth checking with the local authority.
If the building is listed then consent is required. Altering a listed building without consent is a criminal offence.
Similarly, if the building is in a conservation area, then permission is more likely to be required.
The Construction (Design and Management) Regulations (CDM regulations) are intended to ensure health and safety issues are properly considered during a project's development so the risk of harm to those who have to build, use and maintain structures is reduced.
If an extension is large, the regulations may apply, although the client's obligations are likely to be transferred to the contractor on a single contractor project or to the principal contractor on a project involving more than one contractor.
The Party Wall etc. Act 1996 grants the owner of a property the legal right to undertake certain works that might otherwise constitute trespass or nuisance. However, it also protects the interests of adjoining owners by imposing a requirement that they are given prior notice of proposals.
In addition, it provides for a mandatory dispute resolution procedure if neighbouring owners have concerns about the implementation of proposals.
 Design considerations
It is sensible for homeowners considering an extension to define their objectives and prepare a realistic brief, budget and schedule. By consulting architects, contractors and planners early, it should be possible to determine the scope and scale of extension that could be permitted and is affordable.
The fees associated with appointing an architect or design and build contractor may be off-putting to homeowners, however, they tend to be relatively low compared to the construction cost and the experience of professionals is likely to save money and time over the life of the project, as well as resulting in a better-designed end product. Professional input can also help secure permissions.
However, it is important that the design suits the homeowner, not the professionals appointed, and so it is important that time is spent developing a clear brief. The brief should focus on what it is the homeowner wants to do in the extension, rather than how it should be designed, this helps keep options open, as designers may have ideas that would not have occurred to the homeowner.
 Related articles on Designing Buildings Wiki
- Avoiding planning permission pitfalls.
- Basements in buildings.
- CDM for self-builders and domestic clients.
- Conservation area.
- Custom-build homes.
- Hiring an architect as a domestic client.
- How to build a garage.
- How to build a porch.
- How to find a builder.
- Kit house.
- Licence to alter.
- Line of junction notice.
- Listed buildings.
- Party Wall Act.
- Planning permission.
- Self-build homes.
- Self-build home: project plan.
 External references
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