Do you need planning permission for that? Extensions and exempted development

The question almost always arrives as a number. Someone has heard a figure in square metres and wants to know whether their extension fits under it. That is the wrong shape of question, and it is how people end up building something they then have to regularise.

Exempted development is not an allowance you spend up to. It is a class of works that comes with a list of conditions, and the exemption only applies if every condition is met. Miss one and the whole thing needs permission, no matter how far under the floor area you were.

What changed on 27 July 2026

This matters because the rules moved recently, and a lot of advice online still describes the previous position. A package of new exempted development regulations came into operation on 27 July 2026. For extensions, the relevant one is the Planning and Development (Exempted Development (Act of 2000)) Regulations 2026, S.I. 338 of 2026, which increased the exempted floor area for a single-storey extension to a principal dwelling from 40 square metres to 45 square metres.

The same package brought in several other changes to domestic exemptions, including dormer roof boxes and roof lights, external wall insulation, heat pumps no longer being restricted to the rear of the property, cycle and bin storage, an increase in the allowance for a structure in the back garden from 25 square metres to 30 square metres, and new classes covering subdivision of a house and a detached auxiliary dwelling in the rear garden. Those last two carry their own notification requirements and time limits, so treat them as a separate exercise rather than an extension.

This guide states the position as of August 2026. If you are reading it much later, check whether it has moved again.

The conditions attached to the extension allowance

The floor area figure is the headline, not the rule. The conditions that travel with it include:

• The allowance is cumulative. Previous extensions count against it. If the house has been extended before, that floor area comes off what is left, even if the earlier work was done by a previous owner.
• Private open space has to survive. A rear extension cannot reduce the private open space in the rear garden, reserved for the occupants of the house, below 25 square metres.
• Height is tied to the existing house. The walls of the extension are limited by reference to the height of the existing rear or side walls rather than to a free-standing figure.
• Windows have set distances from boundaries, with different distances at ground level and above ground level.
• Above ground floor level is treated far more restrictively. The increased allowance is for a single-storey extension. Anything at first floor level is subject to its own, much smaller limits and its own separation distance from the boundary. If you are thinking about a two-storey extension, do not assume any part of the headline figure applies to the upper floor.

We have deliberately not put numbers on the last three. They are conditions rather than round figures, they read differently depending on whether your house is terraced, semi-detached or detached, and getting one of them wrong is exactly the failure mode this guide exists to prevent. Your local authority’s planning section will confirm them for your house, and it costs you nothing to ask.

Exempt from planning is not exempt from anything else

This is the single most expensive misunderstanding in the area. The Department was explicit when the new exemptions were announced that all relevant Building, Building Control and Fire Regulations still apply. Planning permission and building regulations are two separate systems, and exemption from one says nothing about the other.

In particular, there is now a gap worth knowing about. The building control threshold that triggers the fuller commencement notice process for a domestic extension sits at a total floor area greater than 40 square metres, while the planning exemption now runs to 45. An extension in that band can be exempted development and still require a commencement notice accompanied by general arrangement drawings, a schedule of design documents, an inspection plan and a Certificate of Compliance (Design), with an assigned certifier appointed, unless the owner instead files a declaration opting out of statutory certification. Opting out does not reduce the obligation to actually comply with the Building Regulations. It only changes who certifies compliance.

Talk to your local authority’s building control section as well as its planning section. They are different people answering different questions.

Reasons the exemption may not apply to your house at all

Even where the works fit the class, the exemption can be switched off:

• A condition on a previous planning permission may have removed exempted development rights. This is common on houses in estates built under a single permission, and it is invisible unless you look up the original grant.
• Protected structures and buildings in an Architectural Conservation Area are treated separately, and works that would be exempt elsewhere are frequently not exempt here.
• The works themselves may not be what you think. Converting an attic, changing a garage, or altering the front of the house sit in different classes with different rules from a rear extension.

How to get an answer you can rely on

Advice from a builder, an architect, a neighbour who did the same thing in 2019 or a website is useful for orientation and worthless as authority. The planning authority for your area is the authority, and there is a formal mechanism for asking it.

Under Section 5 of the Planning and Development Act 2000, you can apply in writing to your local authority for a declaration on whether a specific proposal is or is not exempted development. You submit the details and site maps, the planning authority assesses it, and it issues a declaration, generally within four weeks. That declaration is a document you can hand to a solicitor years later, which is more than can be said for a conversation.

It is worth doing whenever you are near a threshold, whenever the house has been extended before, whenever there is any question over a previous permission, or whenever the work is anything other than a straightforward single-storey rear extension. It is far cheaper than discovering the position after the block work is up, and unlike a retention application, it does not start with you already in the wrong.

Before you talk to a builder

Have the planning position settled, or at least understood, before you ask anyone to price the job. A builder quoting on a scheme that later has to shrink to fit an exemption, or stop for a permission, is quoting on something that no longer exists. It is also one of the things a good builder will ask you about first, and being unable to answer tells them something about how the rest of the job is likely to go.