House Extensions near you

Plot geometry does most of the deciding here. The terraced and semi-detached stock that dominates British towns leaves very little room to the side and a long thin garden to the rear, which is why the rear extension is the default and the side infill is the clever one. Start from what the plot allows rather than from a floor plan drawn in the abstract.

Permitted development rights let householders extend without applying for planning permission, provided specific limitations and conditions are met. They apply to houses only: not to flats and maisonettes, not to other buildings, and not to houses created through certain change of use or new dwellinghouse permitted development rights. They are also assessed cumulatively. The rules apply to the proposed extension together with any previous extensions, the total enlargement, measured against the original house, meaning the house as first built or as it stood on 1 July 1948 if it is older than that. A previous owner's extension counts against your allowance.

The Planning Portal summarises the English conditions that apply to all extensions. Only half the area of land around the original house may be covered by extensions or other buildings. The extension cannot be higher than the highest part of the existing roof, nor higher at the eaves than the existing eaves. Where it comes within two metres of the boundary the eaves height cannot exceed three metres. It cannot be built forward of the principal elevation, or of the side elevation where that elevation fronts a highway. The work cannot include verandas, balconies or raised platforms, a microwave antenna, a chimney, flue or soil and vent pipe, or any alteration to the roof of the existing house, and exterior materials must be of a similar appearance to those on the existing house.

Then the limits divide by type. A single-storey rear extension cannot extend beyond the rear wall of the original house by more than four metres for a detached house or three metres for any other house, and cannot exceed four metres in height. A side extension cannot exceed four metres in height, can only be single storey, and can only be up to half the width of the original house. An extension of more than one storey must not extend beyond the rear wall of the original house by more than three metres, must not be within seven metres of any boundary opposite the rear wall, must match the roof pitch of the house as far as practicable, and must have any upper-floor side-elevation window obscure-glazed and non-opening unless the openable part is more than 1.7 metres above the floor.

England has a middle path between permitted development and a full householder application, and it is widely misunderstood as a formality. Where the house is not on Article 2(3) designated land and not on a Site of Special Scientific Interest, the limit for a single-storey rear extension is increased to eight metres for a detached house, or six metres for any other house, subject to prior approval. That means the local planning authority must be informed of the proposed work through a prior approval application.

Prior approval is not the same as planning permission and it is not the same as doing nothing. It is a defined procedure in which the authority considers a narrow set of matters and neighbours are given an opportunity to comment. The other permitted development conditions still apply on top of it: a larger rear extension still cannot exceed four metres in height, still cannot be built forward of the principal elevation, and still counts towards the limit of half the land around the original house.

Because the deeper limits are switched off entirely on designated land and Sites of Special Scientific Interest, where the ordinary four-metre and three-metre limits are what remain, the first question for any deep rear extension is a locational one. Find out what the property sits within before designing to eight metres.

Extension foundations are usually deeper than the neighbour's and usually close to them, which is exactly the situation section 6 of the Party Wall etc. Act 1996 was written for. The section applies where a building owner proposes to excavate, or excavate for and erect a building or structure, within three metres measured horizontally from any part of a neighbour's building or structure, and any part of the proposed excavation, building or structure will within those three metres extend to a lower level than the bottom of the neighbour's foundations. It applies again within six metres, where the excavation would meet a plane drawn downwards towards it at forty-five degrees to the horizontal from the bottom of the neighbour's foundations.

The procedure is precise. Under section 6(5) the building owner must serve notice on the adjoining owner at least one month before beginning to excavate, indicating the proposals and stating whether they propose to underpin or otherwise strengthen or safeguard the neighbour's foundations, accompanied by plans and sections showing the site and depth of the excavation. Under section 6(7), if the adjoining owner does not serve a notice consenting within fourteen days of service, they are deemed to have dissented and a dispute is deemed to have arisen. The notice also lapses under section 6(8) if the work has not begun within twelve months of service and is not prosecuted with due diligence, so serving notices far too early is its own mistake. Work to an existing party structure is covered by a different notice under section 3, which must be served at least two months before the proposed work begins.

A deemed dispute is not a crisis; it is the start of a defined process. Under section 10, either an agreed surveyor is appointed, or each party appoints a surveyor and those two select a third. The surveyors settle the matter by award, the award is conclusive and cannot be questioned in court except as the section provides, and either party may appeal to the county court within fourteen days of the award being served on them. Section 6(3) also gives the building owner the right, and imposes the obligation if the adjoining owner requires it, to underpin or otherwise strengthen or safeguard the neighbour's foundations at their own expense so far as may be necessary. Note the extent of the Act: England and Wales only.

Planning permission and building regulations approval are independent of each other, and an extension that is entirely permitted development will still be inspected by building control. The Building Regulations 2010 cover the construction and extension of buildings, and GOV.UK states directly that building regulations approval is different from planning permission and you might need both. On an extension the controlled elements are the usual ones: foundations, drainage, structure and openings, walls below ground level, thermal performance, ventilation and, where the extension involves them, electrics and gas.

There is one Approved Document that does not follow the extension in, and it is the one homeowners most often assume does. Approved Document M Volume 1 covers access to and use of dwellings, and its own application paragraph says that the recommendations of the volume apply to newly erected dwellings and dwellings undergoing material alteration only, and that they do not apply to the extension of a dwelling. The requirement itself carries the same limit on its face: requirement M4(1), Category 1 visitable dwellings, does not apply to an extension to a dwelling. Where a dwelling is subject to a material alteration, the approved document's position is that the building should be no less compliant with M4(1) than it was before the work took place.

That does not mean an extension can ignore access. It means the standard is set by the occupants and the designer rather than handed down by Part M, so if the extension needs to be usable by someone with limited mobility, the brief has to say so. The requirements for accessible and adaptable dwellings and for wheelchair user dwellings, M4(2) and M4(3), are optional requirements that apply only where a planning condition imposes them on new development.

The Community Infrastructure Levy is a charge that local authorities may adopt to fund infrastructure, and it is the reason a large extension can attract a bill that has nothing to do with the builder. Government guidance states that most new development creating net additional floor space of 100 square metres or more, or creating a new dwelling, is potentially liable, and that in areas where the levy operates it may be payable where the gross internal area of new build is 100 square metres or more. The Planning Portal flags the same threshold on its extensions guidance: if the work adds over 100 square metres of floor space, it may also be liable for a charge under the levy.

There are exemptions, and the one that matters to householders has to be claimed rather than assumed. Guidance lists an exemption for residential annexes and extensions where an exemption has been applied for and obtained prior to commencement of the development, and a separate self-build exemption for whole houses. For residential annexes and extensions, the applicant must be an owner of a material interest in the main dwelling who occupies that dwelling as their sole or main residence. Development of less than 100 square metres is generally exempt as minor development, unless it consists of one or more dwellings.

Two procedural points cause most of the trouble. The exemption must be obtained before development commences, and where an exemption or relief has been obtained, a commencement notice must be submitted before development commences. Guidance states that failing to submit one in time means the charging authority must impose a surcharge equal to 20% of the notional chargeable amount, capped at 2,500 pounds. Since the levy applies only where the local authority has adopted a charging schedule, the starting question is whether yours has.

Relying on permitted development means there is no planning decision notice at the end, which is fine until you sell. A lawful development certificate is the document that fills that gap: an application to the local planning authority for formal confirmation that the work is or was lawful. It is not compulsory and it is not a planning permission, but it converts a judgement that the extension complied with the conditions into a document a conveyancer can accept.

The same logic applies on the building control side. GOV.UK warns that without approval you will not have the certificates of compliance you may need when you want to sell your home, and that the person doing the work could be prosecuted and fined for not complying with the Building Regulations. The completion certificate from the building control body, or a regularisation certificate where approval was obtained retrospectively, is the corresponding document. Regularisation is available only from a local authority building control body, and only for work carried out after 11 November 1985.

Scotland's householder permitted development rights are written in classes. Class 1A covers a single storey ground floor extension, and Scottish Government guidance summarises the effect of the limitations as: extensions generally located to the rear; if the extension is on, or within one metre of, the boundary, it cannot project from the rear wall of the existing dwellinghouse by more than three metres for a terraced house or four metres in all other cases; eaves height a maximum of three metres; overall height no more than four metres; and the footprint no larger than the original dwellinghouse, and not covering more than half the relevant front or rear curtilage. Class 1B covers a ground floor extension of more than one storey, and there the guidance notes that no part of the extension can be within ten metres of any boundary, which it acknowledges means most two-storey extensions will need a planning application. There are no Class 1A rights in a conservation area or for flats, a building warrant will likely be required, and listed building consent is normally needed for a listed building.

Wales sets its own numbers again. Welsh Government guidance states that no extension can be constructed in front of the principal elevation, that no more than half the area of land around the original house can be covered by an extension, and that the extension cannot be higher than the highest part of the roof or higher than the eaves of the existing dwellinghouse. Single storey rear extensions must not extend beyond the rear wall by more than four metres and cannot exceed four metres in height, and note that Wales does not split that figure between detached and other houses the way England does. For rear extensions of more than one storey the ground floor part cannot project more than four metres, the upper floor part more than three metres, and the distance from the rear wall of the extended house to the rear boundary must be at least 10.5 metres. Side extensions cannot make the widest part of the resulting house more than 50% wider than the widest part of the original, and extensions of more than one storey are not permitted in a conservation area, national park, area of outstanding natural beauty or World Heritage Site.

In Northern Ireland, planning is run by the district councils. Official guidance confirms that permitted development rights exist for householders but that the works must meet conditions including dimension restrictions for extensions, that rights are restricted in designated areas, and that you should contact the council planning office for your area to check, and contact building control early, since many domestic extensions and alterations need building regulations approval as well.