Single storey rear extensions

A single storey extension is a great way to add valuable space, but careful planning, designing and budgeting are essential to ensure a successful project.

Under Permitted Development Rights (PD), homeowners can add a single storey extension to their home without applying for planning permission, as long as it adheres to a list of conditions. If your project falls outside of the rules, you’ll need to make a full planning application instead.

PD Rights can be found on the Planning Portal website but, as a guide:

• Single storey extensions can’t exceed 50% of the area of land your house sits on. This excludes the area covered by the house, but doesn’t include any existing additions or outbuildings.
• You can’t extend forward of the main elevation (or the side elevation if that fronts the highway).
• The single storey extension can’t exceed 4m from the original back wall of a detached house, and no more than 3m for terraces and semi-detached properties.
• The single storey extension must measure less than 4m in height, or 3m if it is within 2m of a boundary.
• Building materials must be similar to the existing home.

Planning rules differ slightly in Scotland, Wales and Northern Ireland. Plus, in some areas of the country (known as designated areas) PD Rights are more restrictive – this includes Conservation Areas, National Parks, or Areas of Outstanding Natural Beauty. PD rights do not apply if a property is listed.

If you want to build something bigger under PD, you can push for a large single storey rear extension of up to 8m for detached houses and up to 6m for a semi or terrace via the Prior Approval scheme. This allows neighbours 21 days to object, giving valid reasons. After which, your Local Planning Authority has 21 days to grant approval – you then pay a fee for a Lawful Development Certificate, which proves that your project is legal.

Although a Lawful Development Certificate, which costs £103 in England,  isn’t a legal requirement for all extensions, it can be useful. It essentially proves to your local authority and future buyers that your project was legal at the point of construction.

Apply for full planning permission

If your house extension requires a formal application, which costs £206, investigate whether there are similar finished schemes in your area. Local precedent can influence the outcome of an application. It’s also worthwhile contacting your Local Planning Authority to discuss your proposal before submission (a pre-planning application). Your assigned planning officer will review the proposal and notify you if it contravenes any policies. You won’t get a definitive yes or no, but you will receive useful guidance to improve your chances of gaining consent. The cost of this service varies from council to council but, budget between £50 and £200.

All single storey extensions will need Building Regulations approval and there are two main routes to securing it. A full plans application is when your detailed plans and drawings are checked and approved by your Local Building Control Authority prior to work starting. If your project is complex, this is usually the way to go.

The second route is a Building Notice form, where work can begin 48 hours after your notice has been received by the local authority; it doesn’t require detailed drawings. This route is ideal for straightforward projects. For more information, visit LABC.

Develop the design

An open-plan layout is a common choice for single-storey extension projects. Here, existing internal walls on the ground floor are opened up to connect old and new areas. The most important design considerations are:

1. Ensuring there is a natural flow from the existing house to the rear extension.
2. Making sure the middle of the floorplan doesn’t become too dark if the ground floor is opened up entirely.
3. Make sure accessibility, such as door widths and level thresholds, accommodates your abilities now and in the future.
4. Positioning windows and doors to make the most of views and provide a strong interior/exterior connection.

Select materials

Glass

Well-placed glazing can transform a rear extension. It looks great and allows natural light to flood the interiors, helping to create an open and inviting zone. Take care when positioning glazing on south or west facing walls to ensure it doesn’t cause overheating.

Timber

External wood cladding is a durable and renewable material. It can be treated to preserve its colour or left untreated to weather naturally. It also makes a great internal feature – often used for flooring and carpentry to create a warm finish.

Metal

Steel, zinc, aluminium and copper are all popular cladding materials. All are low maintenance and will help create a sleek, modern aesthetic.

Brick

While brick is often considered to be a traditional material, it can also be used to create innovative modern extensions. From creative brickwork bonds to mortar styles, colours and blends, it’s possible to make a real design statement.

Work out a budget

Prices for a single storey extension will vary significantly, depending on factors such as size, specification and the area you are building in. As a guide, expect to pay from £2,800 per sqm for a standard, cost-efficient extension in London. For a high-end design, expect the cost to rise to around £4,500 per sqm in the capital.

Plan a build schedule

While every house extension is different and schedules can change at a moment’s notice, often due to weather, this planner offers a guide to all the essential stages:

1. Pricing and mobilisation (5 weeks)
   At least four weeks are required for full pricing and a further week for contractor mobilisation.
2. Site preparation (1 week)
   This involves making sure there is clear access to the site, that building materials have been delivered and that machinery is on site.
3. Groundworks (2-3 weeks)
   Builders will arrive to dig foundations, followed by laying pipework, drainage and services. Concrete footings will be poured in and levelled, too. Building Control will need to sign off this stage.
4. Superstructure (3 weeks)
   The extension’s frame is built, insulation fitted and flooring sub structure fitted. Building Control carries out another inspection.
5. External walls (2 weeks)
   Lintels, doors, window frames and wall tiles will arrive ready to be fitted, and new walls are tied to existing ones.
6. Internal walls (1 week)
   Internal walls are built.
7. Roof structure (1 week)
   Your carpenter will start building the roof structure, or your contractor will fit prefabricated roof trusses. Rooflights will be fitted at the same time.
8. Roof covering (1 week)
   Roof battens are cut and fitted over the membrane and the roof covering is laid over the top, along with finishing details, such as flashing.
   
9. Windows, doors & exterior finish (3 weeks)
   All exterior finishes are completed, windows and doors are fitted into the frames already fitted. Guttering and pipework is added and first-fix carpentry, plumbing and electrics completed.
10. Plastering & decorating (3 weeks)
    Interior walls are boarded up and the plaster is left for a minimum of one week before decorating and painting
11. Second fix (2 weeks)
    Second-fix electrics, such as sockets and switches, are put in place and lighting is fitted. Taps are installed and connections completed, flooring is laid and kitchen and bathrooms, if applicable, are installed.
12. Snagging (2 weeks)
    Any issues need to be resolved with the relevant trades as soon as possible.
13. Sign off (1 week)
    If happy that all works comply, Building Control will sign off and provide all the necessary paperwork.

Get professional help

A good architect can be worth their weight in gold when it comes to house extensions. Trained in design and project management, there’s no denying that a professional’s input will leave you with a single-storey extension that is creative, maximises light and space, is made using unique materials, finishes and fittings, and built with sustainability and energy-savings in mind.

An architect will also help you find a suitable contractor, quantity surveyor and engineer, and will guide you through the planning, Build Regs and contractual process. They can, if you so choose, act as project manager, watching over the whole process to ensure your home extension is built to schedule, budget and the correct standard.

Discover Architecture for London’s house extension projects. If you would like to discuss your project please contact us.