How Far Can You Extend Without Planning Permission?
AMP Builder Ltd · Published · Updated November 4, 2026 · Sources checked 2026-11-04
This guide focuses on houses in England, including Harrow. Different rules can apply elsewhere in the UK and to flats, maisonettes or other property types. Confirm the position for your address and proposal.
The usual starting point is 3 metres for an attached house and 4 metres for a detached one. That simple answer leaves out several important conditions, though, and it applies only to single-storey rear extensions.
In this guide
Here’s how the rules work and what to check before you commit to a design.
The basic rear extension limits
Under permitted development, single-storey rear extensions generally can’t go beyond the original rear wall by more than 4 metres on a detached house, or 3 metres on any other house.
These distances are measured from the original rear wall, not necessarily from the wall you see today. If the house has already been extended, that earlier work matters.
Height also matters. A single-storey rear extension must not exceed four metres overall. Where it is within two metres of a boundary, the eaves must not exceed three metres. Other conditions apply, including the relationship to the existing house’s roof and eaves; check the complete guidance for the actual proposal.
What “original house” means
The original house is generally the building as first constructed, or as it stood on 1 July 1948 if it was built before then . A previous owner may have added an extension, and that can reduce what’s left of your allowance.
Check old planning records and your property documents, and ask the council if you’re unsure.
Going further with prior approval
Prior approval applies only to larger single-storey rear extensions, which can go up to 8 metres on detached houses. The equivalent limit for semi-detached and terraced homes is 6 metres.
This isn’t the same as a full planning application, but it isn’t automatic either. The homeowner applies to the local planning authority with supporting information and the correct fee. Neighbours can be consulted, and their objections can affect the outcome .
Don’t start work until the process has concluded and you’ve received the decision you need.
When permitted development won’t cover your plans
You may need a planning application if:
The extension is bigger than the permitted limits.
The design adds more than one storey beyond the rules for that type of extension.
The property is in an area where the rights are restricted.
Earlier conditions removed your permitted development rights.
The extension is at the front of the house, or at the side beyond the relevant limits .
Check any designated-land restrictions and the actual Article 4 Direction for the property. Different directions remove different rights; do not assume one general answer applies to all work.
Planning is only part of the process
Building regulations are separate from planning. Even where planning permission isn’t needed, building regulations approval is usually still required for extending a building.
Also consider:
Party wall duties: These can apply when work is on or near a shared wall or boundary .
Drains: Building near a public sewer may need agreement from the water company .
Light and layout: A deep extension can make existing rooms darker.
Foundations and trees: Ground conditions and nearby trees can affect the design.
A practical checklist
Measure the depth you actually need rather than building up to the maximum.
Check what the house looked like originally.
Confirm any local restrictions.
Sketch how the existing rooms will work with the extension.
Choose your planning route: permitted development, prior approval or a planning application.
Consider a Lawful Development Certificate for evidence .
Arrange building control and any party wall steps.
Compare quotations against the same drawings and specification.
FAQs
Does a conservatory count?
Generally, a previous addition can affect what’s still allowed. Check its status and how it was built.
Is 6 metres always allowed on a semi-detached house?
No. It depends on prior approval and on meeting all the other conditions.
Can my neighbours stop my extension?
Their objection doesn’t automatically decide the outcome, but it can matter in prior approval and planning. Party wall duties are a separate matter.
Prepare for a practical project conversation
Bring the property postcode, your priorities, any relevant plans and a list of the questions you still need answered. Distinguish the work you know is required from the options you want to explore. Ask who will provide the necessary assessment, design and approvals, and keep the agreed scope with your project records. A useful first discussion identifies the next information needed rather than treating an early idea as a fully developed specification.
Explore extensions & structural works or Request a Free Consultation. You can also call AMP Builder on 07460 078657.
Sources and review
Source check: 4 November 2026. This is general information, not a property-specific planning, legal or technical determination. Requirements can change; confirm the current position with the relevant authority or qualified professional before acting.
