Do I need an architect for an extension?

No. There is no legal requirement, anywhere in the UK, to use an architect to design an extension or to submit the planning application for one. You can draw the plans yourself, and plenty of people do. The only thing the law protects is the word: under the Architects Act 1997, nobody may call themselves an architect unless they are on the register held by the Architects Registration Board, which you can search free. The work itself is open to anyone. So the question is not whether you need an architect. It is whether your particular project is the kind where one earns the fee.

That question deserves a better answer than you will get from most of the pages ranking for it, because nearly all of them are written by people selling drawings. We are not. We run a public evidence directory, and our records include 403,748 planning applications that mention an extension, of every type and stage, drawn from council planning registers across the UK. What follows is what those records actually show, and a way to decide that does not depend on taking anyone's word for anything.

First, check whether you need permission at all

The permitted development rules below apply in England only. Scotland, Wales and Northern Ireland have their own rules with different limits. Check with your council before relying on any of them.

In England, many extensions are permitted development: they need no planning application at all, provided they stay inside limits set out in the legislation. For a single-storey rear extension the basic depth limit is 4 metres for a detached house and 3 metres for any other house, with a height limit of 4 metres and an eaves limit of 3 metres when the extension comes within 2 metres of a boundary. A larger single-storey rear extension, up to 8 metres detached or 6 metres otherwise, can be possible through a prior approval process, which means notifying the council and letting neighbours comment first, not just building. Two-storey rear additions can qualify within tighter rules: no more than 3 metres beyond the rear wall and at least 7 metres from the rear boundary. Side extensions must be single storey, at most 4 metres high and no wider than half the original house. And in every case, extensions and outbuildings together may not cover more than half the garden.

Three caveats matter more than the numbers. Flats and maisonettes have no householder permitted development rights at all; these rules are for houses. Extra limits apply in conservation areas, national parks and similar designated land, and some councils have removed permitted development rights locally with an Article 4 direction. And even when no planning permission is needed, you can apply to the council for a lawful development certificate, which is optional but gives you dated proof the work was lawful, the piece of paper a buyer's solicitor will one day ask for.

One thing is never optional: building regulations. As the Planning Portal's guidance sets out, almost every extension needs building regulations approval whether or not it needs planning permission. That is a separate process, run through building control, covering the structure, insulation, drainage and safety of what you build. Skipping it harms you twice, once in safety and once at sale time.

What actually happens to extension applications

If your project does need an application, the record is reassuring. Of the 126,954 Full extension applications decided in the last five years in our records, 90.1% were approved. That number tells you how contested the terrain is, not how your application will do: approval rates describe decided applications, not any one proposal's odds. Two more honesty notes belong next to it. Withdrawn applications are not decisions and sit outside these figures, about one in twenty of concluded extension applications ends that way, and schemes that were never submitted are invisible to planning records entirely. And while we say extension applications rather than home extensions, that is deliberate: a small share of what we match, roughly 3 in 100, are extensions to schools, offices, farms and other non-homes.

The gap in that chart is the most useful fact on this page. Two-storey extensions are approved less often than single-storey ones, 84.5% against 91.5%, and the reason is what they are: bigger, more visible proposals that affect neighbours more, drawn from a harder mix of sites. It does not measure who drew the plans, and it is not a reason to expect refusal. It is a reason to take the design seriously, because at two storeys the council will. And if you live in London, expect harder terrain: borough extension approval rates there ran from 66.7% to 92.7% over the same five years, brackets around the national figure rather than a contradiction of it.

How long the wait is

Of extension decisions made in the last five years, half came within 61 days of the application appearing in the council's record, and 9 in 10 within about 129. The 8 week statutory target for minor applications in England and Wales was met for 42.3% of the 126,937 decisions with usable dates, and the single most common day for a decision to land is day 56, the target itself. Decisions cluster there because that is the system working to its deadline. Extension applications actually run slightly quicker than full applications of every kind: our planning permission guide measures all full applications, where the typical wait is 62 days all time and 65 in the last five years, while this page counts only extensions, at about 61. Same records, different slice.

When an architect earns the fee

In councils that publish who handled the paperwork, about two thirds of decided extension applications listed an agent: an architect, architectural technologist, planning consultant or the builder's office. In the rest of the country the records don't say. What the records cannot tell you, in either direction, is whether professional drawings change approval odds, because applications with an agent are a different mix of projects from those without. Experience and common sense have more to offer on where design input has the most room to matter: two-storey work, anything in a conservation area or near listed buildings, awkward plots and party wall situations, and the projects you will live inside for decades, where the difference between an adequate space and a good one is the whole point of spending the money.

If that is your project, the AI era hands you one real advantage: you no longer have to take any firm's word for their experience. An architect's planning record is public. For firms in our records, their profile here shows the applications they have handled, in which councils, and how those applications were decided. A firm that knows your council's conservation officers by name will show it in the record. Check before you shortlist, not after.

The other routes, honestly

For a straightforward single-storey extension on an unremarkable plot, several routes are entirely legitimate, and pretending otherwise would be selling you something. A Chartered Architectural Technologist, MCIAT through the Chartered Institute of Architectural Technologists, specialises in exactly this kind of technical design work. An architectural technician or an experienced drawing service can produce council-ready plans. Many design-and-build builders carry their own drawing experience and take the whole job end to end. And drawing it yourself is legal, if you have the patience for the drawing conventions councils expect. On cost, the HomeOwners Alliance puts planning drawings at roughly 1,000 to 2,500 pounds for a single-storey extension and 2,000 to 3,500 for two-storey, rising to several thousand for a full service through building regulations; fee structures vary, so get the scope in writing. Whoever you shortlist, the checks are the same and they are free: dated reviews, company record, and past planning applications.

Questions homeowners ask