You are currently viewing Building an extension

Building an extension


Building an addition to a house is becoming a more common extension for homeowners looking to optimize the space available on their property. With taxes and stamp duty to consider, as well as the stress of buying a new house and rising property prices, an extension may appear to be a more cost-effective alternative.

The type and scale of a house extension is determined by the homeowner’s needs, money, and the site’s viability.

Extensions of various kinds


A garage is a common type of single-story extension that is constructed to connect to an existing building and can be helpful for storing a vehicle, among other things.

See How to Build a Garage for more details.


This is a single-story addition to a house’s front. They may not require planning authorization due to their small size (although they may in a conservation area or for a listed building). If the porch is not separated from the house by an internal door and is heated, or if there are structural, accessibility, or drainage considerations, building rules approval may be necessary.

See How to Build a Porch for more information.


These are simple one-story constructions with windows and a UPVC frame, although they can also be made of wood or aluminum. They might have a low-level brick wall around their perimeter, which the framework will be built upon. Whether or not planning permission or building regulations approval is necessary depends on the type and size of the conservatory.

For additional information, go to Conservatory.


An orangery, sometimes known as a sunroom, is similar to a conservatory but includes a solid roof and walls in addition to glazing. This makes them more expensive than a conservatory, but they are also more durable and efficient in retaining heat. As with conservatories, planning authorization and building regulations apply.

Extension with only one floor

A single-story extension is built next to the existing house. The method of connection must take into account a variety of factors, including openings between the extension and the old building, roof structure joints, flue and drain placements, and so on. In some cases, planning authorization is not required, but building regulations approval is.

Extension with two or more stories

The same principles apply as for a single-story extension, though the joints and structure will almost certainly be more involved, and planning approval will almost certainly be necessary.

Extension of the structure

An extension is built on top of an existing structure, most typically a garage. Because most garages have only single-skin brickwork, new foundations and inner leaf wall construction may be required to ensure stability. Because the condition of foundations cannot be guaranteed, an appropriate evaluation is required to determine the amount to which additional work is required.


A basement is a portion of a building that is partially or entirely below ground level.

Basements are being constructed or converted for living space in places such as London because to the high demand for housing, the high cost of land, and the high cost of moving. This is raising worry in some regions, where very big, multi-story basements are being constructed, causing major disruption to neighbors over time. As a result, some planning constraints have been implemented. For further information, see Basements in Buildings.

The planning situation will be determined by the specific nature of the proposed development, hence it is advisable to seek pre-application guidance from the Local Planning Authority.

Building codes apply to new basement construction and address issues such as ventilation, drainage, ceiling heights, damp proofing, electrical wiring, water supplies, and methods of escape, among others.

For further information, see Basement excavation.

Converting a loft

Loft conversions can raise the value of a home by up to 20% while also providing up to 30% extra living space.

Loft conversions are frequently possible as Permitted Developments, which do not require planning permission. If the proposals do not fall within these parameters, however, planning clearance will be necessary. If the house is in a conservation area or is a listed building, planning clearance may be required.

Planning permission is required.

Extensions and house expansions may not require planning authorization because they are deemed allowed developments. However, the extension is subject to the following restrictions and conditions:

  • Extensions shall not exceed up more than half of the total land area surrounding the ‘original house’ (the house as it was first built or as it was on 1 July 1948).
  • The eaves and ridge heights must not exceed the height of the existing house.
  • The maximum height of a single-story back extension is 4 meters.
  • Multi-story rear extensions are not allowed to exceed the length of the back wall by more than 3 meters.
  • The length of a single-story back extension cannot exceed 3 m (if an attached house) or 4 m (if an unattached house) (if a detached house).
  • Two-story extensions can’t be closer than 7 meters to the site’s back fence.
  • Extensions with more than one storey must, to the extent possible, match the existing house’s roof pitch.
  • The materials utilized must match the current building in appearance (not relevant for conservatories).
  • Extensions ahead of a house’s front or side elevations that face a roadway are not allowed.
  • There are no balconies or verandas allowed.

Planning approval will be required if these conditions are not met. Local variances do exist, and Article 4 directives can be used to withdraw approved development rights on a local level, therefore it’s always a good idea to double-check with the local government.

If the building is listed, permission is required. Changing a listed building without permission is a crime.

Self-builders and domestic clients can benefit from CDM

The Construction (Design and Management) Regulations (CDM regulations) are designed to guarantee that health and safety issues are appropriately considered during the development of a project, reducing the risk of injury to individuals who must build, use, or maintain structures.

The requirements may apply if the extension is significant, albeit the client’s obligations are likely to be shifted to the contractor on a single contractor project or to the primary contractor on a multi-contractor project.

For further information, see CDM for self-builders and domestic clients.

Act to Create a Party Wall

The Party Wall etc. Act of 1996 gives a property owner the legal permission to carry out some works that would otherwise be considered trespass or a nuisance. It does, however, protect the interests of adjacent property owners by requiring that ideas be presented to them in advance.

It also includes a mandated dispute resolution procedure in the event that neighboring property owners have issues about the proposals’ implementation.

For further details, see the Party Wall Act.

Considerations for the design

It’s a good extension for homeowners thinking about adding on to their home to identify their goals and create a realistic brief, budget, and timeframe. It should be easy to identify the scope and scale of extension that can be approved and is economical by consulting architects, contractors, and planners early on.

The costs of hiring an architect or a design-build contractor may seem high to some homeowners, but they are typically low when compared to the cost of construction, and the experience of professionals is likely to save money and time over the course of the project, as well as result in a better-designed end product. Permissions can also be secured with the help of a professional.

However, it is critical that the design suits the homeowner rather than the specialists hired, therefore effort should be spent defining a clear brief. The brief should be focused on what the homeowner wants to accomplish with the extension rather than how it should be designed; this keeps alternatives open because designers may have suggestions that the homeowner would not have considered.