Extending a house outwards or upwards

Many single-storey extensions on houses in England do not need a planning application because they fall under permitted development. The Planning Portal summarises the key limits: a single-storey rear extension may extend up to four metres from the original rear wall of a detached house and three metres for other houses, and must not be more than four metres high. Through the prior approval process, and outside designated land such as conservation areas and national parks, those depths rise to eight and six metres.

Extensions and other buildings may not cover more than half the land around the original house. Where an extension is within two metres of the boundary, its eaves may not be higher than three metres. Side extensions are single-storey only and up to half the width of the original house. Extensions of more than one storey may not go more than three metres beyond the rear wall and must be at least seven metres from the rear boundary. Materials should look similar to the existing house.

Original means the house as first built, or as it stood on 1 July 1948. A previous owner's extension counts against your allowance. These rights do not apply to flats and maisonettes, and they may have been removed by an Article 4 direction or a planning condition.

Source: planningportal.co.uk

Building upwards on a house in England can also be permitted development, but only with the council's prior approval and only for houses built between 1 July 1948 and 28 October 2018. One storey may be added to a single-storey house and up to two to a house with more than one storey. Each added storey is limited in height, the house may not exceed 18 metres in total, and windows are not allowed in side walls or roof slopes. Houses in conservation areas and other designated land are excluded.

An extra storey puts load on walls and foundations that were not designed for it. A structural engineer has to check them, and strengthening may be needed. The fire escape route from the new floor is a key Building Regulations issue, and the stairs have to be extended safely into the space.

Source: planningportal.co.uk

Under the Party Wall etc. Act 1996, in England and Wales, work on a party wall needs at least two months' notice, and excavating for foundations within three or six metres of a neighbour's building, depending on depth, needs at least one month's notice. Neighbours cannot stop lawful work, but disputes go to surveyors, and starting without notice can lead to an injunction.

Rear extensions often sit over drains. The Planning Portal points out that drains and sewers may be shared with neighbours or owned by the water company, so ownership has to be clarified first; if a public sewer runs under the site, speak to the water company before the design is fixed. If an extension adds more than 100 square metres of floor space, the Planning Portal notes that it may be liable for the Community Infrastructure Levy, where the council charges it.

Source: gov.uk

Every extension needs Building Regulations approval, even where no planning application is needed. Building Control checks foundations, drainage, structure, fire safety, ventilation and energy efficiency. Extensions with large glazed areas, such as glass roofs and wide bi-fold doors, can need a calculation showing that the extra heat loss is compensated elsewhere.

Knocking through into the existing house needs steel beams or lintels designed by an engineer. Where the extension covers a window or door that the house relied on for escape or ventilation, the design has to replace it.

• drawings, planning route and who submits prior approval or applications
• foundation design, and what happens if ground conditions differ
• structural steel and knock-through details from an engineer
• drainage, and any agreement needed from the water company
• party wall notices and who serves them
• roof, glazing, heating extension and the finishes included

Tell us about the house and the extension you have in mind, and whether you already have drawings or planning. Builders who work locally see the request, not who sent it. One builder can buy it and pays Offrano; it costs you nothing. Before your details go to that builder, we email you its name, address and phone number.

We cannot promise that a builder takes the request; if none does within 21 days, it is withdrawn. You never have to accept a quote. Offrano only confirms that the buyer is a registered business, so ask to see extensions it has completed and check that Building Control signed them off.