Planning permission for house extensions is one of the areas where homeowners are most frequently misinformed — either by well-meaning neighbours who confuse what they have heard with what the rules say, or by contractors who oversimplify to avoid complication. The actual rules are specific, consistent, and learnable. Understanding them before appointing an architect saves time, money, and the frustration of designing to the wrong brief.
Permitted Development — What It Covers and What It Does Not
Permitted development rights allow certain types of development to proceed without a planning application. For single-storey rear extensions in England, the PD limits are: no more than 3 metres from the original rear wall on a semi-detached or terraced house (4 metres on a detached); maximum eaves height of 3 metres; maximum overall height of 4 metres; materials similar in appearance to the existing house; and no extension beyond the side elevation of the original house at the front. These limits apply to the original house as built — not as it stands after previous extensions, which may have used part of the allowance.
"Permitted development is calculated from the original dwelling as built or as it stood on 1 July 1948 — whichever is later. A house that has already been extended uses the original footprint as the baseline, not the current one. This means the remaining PD allowance may be less than the homeowner assumes if a previous extension has already used part of it. Checking the planning history of the property before designing to PD limits avoids this problem."— Planning Portal — Permitted Development for Extensions
The Prior Approval (Larger Home Extension) Scheme
The prior approval scheme allows single-storey rear extensions up to 6 metres (semi/terrace) or 8 metres (detached) in depth — double the basic PD limits — provided the local planning authority is notified in advance and no neighbouring objection leads to a refusal. This is a notification process, not a full planning application: it considers only the impact on neighbouring amenity, not design or materials. Most prior approval notifications proceed without objection. The scheme applies only to single-storey extensions — two-storey extensions at any depth require a full planning application.
When Full Planning Permission Is Needed
A full planning application is required when: the extension is two storeys; the project exceeds the PD or prior approval volume limits; the property is in a Conservation Area or has PD rights removed; the extension would project beyond the principal elevation on the side; or the project involves a change of use of the building. The planning application process in England has a statutory eight-week determination period for householder applications. The fee is currently £258 for a householder extension (England, 2024 — confirm current fee at application).
"The most reliable way to confirm whether a specific extension requires planning permission is a pre-application enquiry to the local planning authority, or a Lawful Development Certificate application confirming the project falls within PD. Both are cheaper than submitting a planning application for a project that does not need one — and a Lawful Development Certificate provides a formal record of the planning position that protects the homeowner on any subsequent sale."— GOV.UK — Lawful Development Certificate
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