Do You Need Planning Permission at All?

The most common misconception about rear extensions is that planning permission is always needed. Very often, it isn't. A large share of single and two-storey rear extensions in England fall under permitted development rights — a national grant of planning permission that comes with conditions attached. Meet every condition and you can build without submitting a planning application, although you will almost certainly still need Building Regulations approval.

The catch is that those conditions are detailed and they interact. A scheme can satisfy five of them and fail the sixth, which pushes you into a full householder application. Before you fall in love with a design, have your designer or a planning consultant run a permitted development check against your specific property — it takes an hour and can save months.

How Far You Can Go Under Permitted Development

For a house — not a flat or maisonette, which have no permitted development rights at all — the headline limits for rear extensions are broadly these:

• Depth: a single-storey rear extension can project up to 4 metres from the original rear wall of a detached house, or 3 metres for a semi-detached or terraced house.
• Two storeys: a two-storey rear extension is normally capped at 3 metres deep, and its rear wall must sit at least 7 metres from your rear boundary.
• Height: single-storey extensions must not exceed 4 metres, and eaves must not be higher than those of the existing house.
• Materials: these should match the existing house as far as is practicable, including brick, roof tiles and window frames.
• Coverage: extensions, outbuildings and other additions together should not cover more than half the land around the original house.

On top of this, two-storey extensions should generally have a roof pitch to match the existing house, upper-floor side windows must be obscure-glazed and non-opening, and nothing may project forward of the principal elevation. Where a scheme exceeds the standard depth limits, a prior approval route has allowed larger single-storey extensions — up to 8 metres for detached houses and 6 metres for others — but this right has been time-limited, so confirm the current position before you rely on it. That route also formally notifies your neighbours.

Boundary Rules and the Seven-Metre Question

Boundaries matter far more than most homeowners expect. The 7-metre rule for two-storey extensions is a blunt instrument designed to protect a neighbour's light, outlook and sense of space, and it applies regardless of how generous your garden looks on a plan. Measure from the original rear wall, not from a previous extension, and measure the shortest distance to the boundary.

Other boundary points worth checking early:

• Extensions must not run forward of the principal elevation, which usually faces the road.
• Side extensions are limited in width, and in designated areas they may not be permitted at all.
• If your extension would sit across a shared drainage run or an easement, you may need consent from the utility provider or the benefiting neighbour.
• Restrictive covenants on older titles can prohibit building even where planning rules allow it.

Your deeds, not the planning system, govern ownership of the boundary itself. Sort that out before foundations are dug.

Neighbours, Notices and the Party Wall Act

Even when no permission is required, talking to neighbours before work starts is simply good practice — and it is far easier than managing a dispute later. Where prior approval is needed, their views are formally sought and can influence conditions, though a reasonable scheme is rarely refused on neighbour objection alone.

The Party Wall etc. Act 1996 is entirely separate from planning. If you plan to excavate within 3 metres of a neighbour's structure and go deeper than their foundations, or work directly on a party wall, you must serve written notice. Allow roughly two months for excavation notices and one month for party structure works. Ignoring this can land you with an injunction and a delay you did not budget for.

When Permitted Development Doesn't Apply

Several circumstances remove your permitted development rights entirely or in part. Flats and maisonettes never have them. Properties in conservation areas, National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites lose various rights — often including two-storey rear extensions and side extensions. An Article 4 direction imposed by the local authority can strip rights from a whole street or neighbourhood. Listed buildings need listed building consent as well as planning permission. New-build estates sometimes had rights removed by condition when the original permission was granted.

A quick check with the planning department's duty officer will confirm which of these applies to you, and it is worth getting in writing.

Building Regulations and Other Approvals

Planning permission and Building Regulations are two different systems, and passing one does not satisfy the other. Building Regulations cover structure, fire safety, thermal performance, drainage, ventilation and electrical work, and you should appoint an approved inspector or use the local authority's building control team before work begins. Expect inspections at foundation, damp-proof course, roofing and completion stages.

It is also worth applying for a Lawful Development Certificate if you are building under permitted development. It is a modest fee and a short form, but it gives you a document proving the extension is lawful — invaluable when you sell. Finally, check for tree preservation orders, obtain freeholder consent if you lease, and confirm whether any shared drainage requires a build-over agreement. Get the paperwork settled before the digger arrives, and the build itself becomes a great deal calmer.