What drawings do I need for a single-storey side extension in London?

The permitted development tests for a side extension

Class A of Part 1 to the General Permitted Development Order 2015 sets the limits. Paragraph A.1(j) applies to any enlargement that would extend beyond a wall forming a side elevation of the original dwellinghouse: it may be no more than one storey, no higher than 4 m, and no wider than half the width of the original house. The government's technical guidance says the width of the original house is calculated at its widest point, and that original means the house as it stood on 1 July 1948, or as first built if later.

Two smaller points trip people up. The guidance treats as a side wall any wall that cannot be identified as a front wall or a rear wall, so a house with an angled corner may have more side walls than you expect. And paragraph A.1(i) rules out eaves higher than 3 m where the extension is within 2 m of the boundary, which a side extension nearly always is. That is why we draw the boundary section to a known datum rather than sketching the roof.

Planning application or lawful development certificate

If the extension passes every Class A test, you do not need planning permission, but many homeowners still apply for a lawful development certificate so a buyer's solicitor can see proof later. Under section 192 of the Town and Country Planning Act 1990, the council must issue a certificate if it is satisfied that the works would be lawful if instituted or begun at the time of the application, and refuse it in any other case. Our lawful development certificate costs guide explains that route in full.

If any test fails, the drawings go in as a householder application instead, and the officer weighs the design against the council's policies. Our householder planning drawings guide covers what that application needs beyond the plans. The drawings we prepare are the same either way, so a certificate refusal does not mean starting again.

Conservation areas remove the side extension right

In a conservation area the answer is simple: planning permission is always needed. Paragraph A.2(b) of Class A says that on article 2(3) land, which includes conservation areas, no enlargement may extend beyond a wall forming a side elevation of the original dwellinghouse under permitted development, at any width or height.

In those streets the officer looks hard at materials and at the gap to the neighbour, so we draw the brick, roof finish and window frames on the elevations and show the street view. Some councils also use Article 4 directions on streets outside conservation areas, which is why we check the council's list for your road before quoting.

How this differs from a side return or a two-storey side extension

A side extension and a side return are often confused. A side return infill fills the narrow passage beside the back addition of a Victorian terrace, and we cover that work on our side return extension drawings page. The extension in this guide sits beside the main body of a semi-detached or detached house, often on a driveway or a strip of side garden.

Adding a second storey changes the route entirely, because Class A allows no more than one storey beside a side wall. Our two-storey side extension plans page covers the set-back and stepped roof that planning officers look for. Where a side extension wraps round the back of the house as well, both sets of limits apply together, as our wraparound extension planning guide explains.

Party wall notices and building control

A side extension built up to the boundary usually brings in the Party Wall etc. Act 1996. Under section 1, an owner who wants to build a wall on the line of junction must serve notice on the neighbour at least one month before he intends the building work to start. Section 6 adds a notice for foundations dug within a distance of three metres measured horizontally from any part of a building or structure of an adjoining owner where they go deeper than the neighbour's own.

Planning drawings are not enough to build from. Building control needs a further set with the foundations, steel over the knock-through, insulation and drainage, which our guide to building control drawings for an extension sets out. Our house extension drawings service covers planning, building regulations and party wall drawings from one survey.

Where the new wall will sit on or right next to the line, building an extension up to the boundary sets out the rules that apply.