3m or 4m? Rear Extension Planning Rules for UK Homeowners
Most UK homeowners can add a single-storey rear extension under permitted development: 3 metres for most houses, 4 metres for detached homes, without needing planning permission at all. If you want to go bigger, up to 6 metres or 8 metres, you can apply for prior approval, provided your home is not on designated land. Building regulations still apply regardless of your planning route, so check your local planning authority’s position before anyone picks up a tool.
TL;DR:
Detached houses can extend up to 4 meters without prior approval, while other homes are limited to 3 meters, with longer extensions possible if on non-designated land.
The total new addition, including previous extensions, must not cover more than half of the property’s curtilage, which limits available space for large rear extensions.
Larger extensions up to 6 or 8 meters can be approved through a 42-day prior approval process, provided the property isn’t on designated land or within Article 4 areas.
Building regulations approval remains mandatory regardless of permitted development rights, especially for structural, safety, and drainage standards.
Local restrictions like Article 4 directions and design guidelines significantly affect permitted development rights and should be checked before planning extension designs.
Table of Contents
Permitted development: exact single-storey rear extension rules and limits
Larger single-storey rear extensions: prior approval explained
Designated land, Article 4 directions and other restrictions
Building regulations, party wall and other statutory consents
Neighbour consultation and prior approval: a step-by-step procedure
Author’s perspective: what matters most before you order work
Permitted development: exact single-storey rear extension rules and limits
Permitted development rights let you build a single-storey rear extension without a planning application, provided you stay within the limits set out in Class A of the GPDO. These rights exist so straightforward, well-proportioned extensions do not need to queue through a full planning process, but the limits are precise and worth checking against your own house before you commit to a design.
The depth limit depends on your house type. A detached house can extend up to 4 metres from the original rear wall without prior approval; any other house type, including semi-detached and terraced, is limited to 3 metres. These are the thresholds confirmed in the permitted development technical guidance, and they apply to single-storey additions only.
Height matters just as much as depth. The overall height of a single-storey rear extension cannot exceed 4 metres, and where the extension sits within 2 metres of a boundary, the eaves height is capped at 3 metres. These limits exist to protect neighbouring light and outlook, and they catch out homeowners who design for depth first and height second.
There’s also a cumulative rule that trips people up: total enlargement of the original house, including any previous extensions, must not cover more than half the curtilage. If a side extension or conservatory already exists, your new rear extension has to be measured against what’s left of that allowance, not treated as a fresh calculation.
Before you draw anything, confirm these points against your own plot:
Your house type (detached versus attached) sets your depth limit at 3m or 4m.
Overall height cannot exceed 4m under permitted development.
Eaves height is capped at 3m within 2 metres of any boundary.
Previous extensions count toward the 50% curtilage coverage limit.
Where an extension joins onto an earlier enlargement, the guidance requires your local authority to assess the total addition together, not the new section alone. That means each extension you add erodes what’s left of your permitted development entitlement, so a home that’s already had a conservatory or side return added has less room to work with on a rear extension.
Larger single-storey rear extensions: prior approval explained
If 3 metres or 4 metres isn’t enough, the prior approval route under Class A.1(g) and A.4 lets you extend further, up to 6 metres for most houses and 8 metres for detached houses, without a full planning application. This only applies where your home sits outside designated land such as conservation areas or AONBs.
The council has 42 days to determine a prior approval notification once it’s validly submitted, under the statutory technical guidance. That timeframe gives you a reasonably predictable path to a decision, far shorter than a standard planning application in many cases.
The process runs in a fixed sequence:
You submit a written description, measurements and a plan showing the proposal, along with the addresses of adjoining properties.
The council serves notice on your neighbours, who then have at least 21 days to raise objections.
If no objections are raised on amenity grounds, the extension can usually proceed without further assessment.
If objections are raised, the council assesses impacts such as loss of light, overshadowing and privacy before issuing a decision.
You wait for a formal decision, or for the 42-day period to lapse, before starting any work.
That last step matters more than it sounds. Starting work before the council notifies you, or before the 42 days expire, removes the legal protection the prior approval process is meant to give you, and can expose the build to enforcement action later.
Two-storey and other non-single-storey extension rules
Extensions of more than one storey sit under tighter permitted development limits, and in practice, many of them still need full planning permission. Under the relevant PD provisions, a two-storey rear extension is commonly restricted to a maximum depth of 3 metres and must not come within 7 metres of the rear boundary, figures that rule out this route for a lot of terraced and semi-detached plots.
Local authority guidance shows why these schemes attract more scrutiny than single-storey additions. Design guidance published by Hounslow sets out the tests planning officers apply, and the recurring concerns are:
Over-dominance, where the extension overwhelms the scale of the original house.
Overshadowing of neighbouring gardens or windows caused by added height or depth.
Loss of outlook for neighbours whose main windows face the new structure.
First-floor projections, often limited to around 2.5 metres under local guidance, even where the ground floor goes further.
How strictly these tests bite depends heavily on your house type. A detached property with generous side gaps has far more room to add a second storey without affecting a neighbour’s amenity than a mid-terrace house, where a rear addition sits hard against boundaries on both sides. End-of-terrace and semi-detached homes tend to sit somewhere between the two, and local design guidance is the most reliable indicator of how your council will weigh these factors in your specific street.
How measurements and definitions actually work
Getting the numbers right starts with knowing what “original house” means: it’s the house as it was first built, or as it stood on 1 July 1948 if it existed then, not as it stands today after any previous owner’s alterations. Every depth and height limit is measured against that baseline, not against your current rear wall if it’s already been extended.
Measurement itself follows specific rules, not estimation:
Projection is measured from the base of the original rear wall, excluding guttering and other minor overhangs.
Where a previous extension has already moved the rear wall outward, your new depth allowance is measured from the original wall, with the earlier addition counted into the total.
“Curtilage” means the land immediately around the house, including garden and outbuildings, and the 50% coverage rule applies to that whole area, not just the footprint of the house.
Stepped rear elevations and outriggers are common measurement pitfalls, since different parts of the same wall can have different original depths.
Getting this wrong is one of the most frequent reasons homeowners discover, partway through a build, that they’ve exceeded their permitted development rights without realising it.
Designated land, Article 4 directions and other restrictions
Permitted development rights don’t apply uniformly everywhere, and a sizeable number of homes sit on land where the rules above are reduced or removed entirely. Designations that commonly restrict or remove PD rights include:
Conservation areas, where external alterations face tighter controls.
Areas of Outstanding Natural Beauty (AONB) and National Parks.
Sites of Special Scientific Interest (SSSI).
Listed buildings, where internal and external changes usually need separate consent.
Land defined under Article 2(3) of the GPDO, which covers several of the categories above.
On top of national designations, individual councils can impose an Article 4 direction that withdraws specific permitted development rights in a defined area, sometimes down to a single street. Guidance on Article 4 directions explains how these work and why checking your local authority’s website before assuming PD rights apply is worth the ten minutes it takes.
Pro Tip: Search your council’s planning policy pages for “Article 4” alongside your postcode before you finalise any extension design.
The practical effect is simple: a rear extension well within the 3m or 4m limits can still need full planning permission if your property sits on designated land or within an Article 4 area.
Building regulations, party wall and other statutory consents
Planning permission and building regulations are two separate systems, and permitted development status has no bearing on the second one. Building regulations cover structural stability, fire safety, thermal efficiency and drainage, and building control guidance confirms that approval is required for extensions regardless of whether planning permission was needed.
Alongside building regulations, several other consents can apply:
A Party Wall Act notice is usually required if your extension involves work on or near a shared wall or boundary, giving neighbours formal notice and a route to raise concerns before work starts.
Listed building consent is separate from planning permission and applies even to minor changes on a listed property.
Highways consent may be needed if your works affect a dropped kerb, footway or drainage connection on the public highway.
Utility diversions sometimes require separate approval from the relevant provider where pipework or cabling crosses the build area.
If your project involves structural calculations for openings, beams or foundations, a structural engineer typically produces the drawings your building control body will want to see; firms offering remote structural and civil engineering services can handle that documentation where you don’t already have one lined up.
Neighbour consultation and prior approval: a step-by-step procedure
The prior approval process for larger single-storey rear extensions follows a fixed sequence, and knowing each step in advance avoids the most common delays.
Prepare your documentation: a written description of the works, measurements, a site plan and the addresses of adjoining properties.
Submit the notification to your local planning authority along with any required fee.
The council serves notice on your neighbours, starting a minimum 21-day window for objections.
If no amenity objections arise, the council can confirm prior approval is not required and you may proceed.
If objections are raised, the council has up to 42 days from submission to issue its decision, under the technical guidance.
The outcome falls into one of three categories: no prior approval required, meaning you can proceed; approval granted, sometimes with conditions on materials or boundary treatment; or approval refused, usually on amenity grounds such as loss of light. Starting work before you receive a decision, or before the 42 days lapse, removes the legal cover the process provides and can leave the extension open to enforcement.
Keep a copy of your submission confirmation and every piece of correspondence with the council. If a dispute arises later over whether the extension was lawfully built, that paper trail is what proves you followed the correct procedure.
Design choices that reduce the risk of refusal
Good design does more to protect a rear extension application than any amount of paperwork. Planning officers consistently favour extensions that read as subordinate to the original house rather than competing with it, and that principle shapes most of the detailed guidance we see from councils.
Practical choices that tend to work in your favour:
Keep the extension’s height and footprint clearly smaller than the host dwelling, with set-backs from side boundaries where possible.
Match brick, render or roof materials to the existing house rather than introducing a contrasting finish.
Use a shallow roof pitch and limit eaves height near shared boundaries, as local guidance from authorities such as Hounslow recommends.
Fit high-level, obscured glazing on side elevations facing neighbouring gardens to reduce overlooking concerns.
Avoid full wrap-around coverage where it would block light to an adjoining garden or window.
Pro Tip: Where your proposal sits near the edge of what’s acceptable, ask your council for a pre-application meeting; many offer informal advice for a modest fee, and it’s far cheaper than a refused application.
Practical checklist before you start
Before any physical work begins, run through a short set of checks that cover planning status, documentation and the professionals you’ll need.
Confirm your permitted development eligibility directly with your local planning authority, rather than relying on general online guidance alone.
Check whether your property sits on designated land or within an Article 4 area that removes or restricts PD rights.
Get an up-to-date site plan and scaled drawings that show the original house, any previous extensions and your proposed addition.
Appoint a structural engineer if your design includes new openings, beams or altered foundations.
Submit your building regulations application alongside, or shortly after, your planning or prior approval route.
Serve any required Party Wall Act notices to neighbours before work starts.
Build a simple neighbour contact list so you can notify them informally, even where formal consultation isn’t required.
Timings vary, but as a general guide: prior approval decisions are due within 42 days, standard householder planning applications typically take around eight weeks, and building regulations approval timescales depend on the complexity of your design and how quickly queries are resolved.
If you’d rather hand the planning and build process to a team that handles it daily, we offer single and double storey extensions with detailed quotes delivered within 24 hours, and live virtual tours of active projects so you can see real workmanship before committing to a contract.
Author’s perspective: what matters most before you order work
We’d urge you to check Article 4 restrictions and local design policy before anything else: it’s the single most common reason a permitted development assumption turns out to be wrong. Permitted development never removes your building regulations or Party Wall Act obligations, whatever size extension you’re planning. If measurements or boundaries feel uncertain, that’s the moment to bring in a professional rather than guess.
— Florin
FAQ
How big can a rear extension be under permitted development?
A single-storey rear extension can extend 3 metres from the original rear wall for most houses, or 4 metres for a detached house, without needing planning permission, under the technical guidance on permitted development. Larger extensions up to 6 metres or 8 metres are possible through the prior approval neighbour consultation process, provided the property isn’t on designated land.
How much does a single-storey extension typically cost?
Costs vary significantly depending on size, specification and location, so there’s no single figure that applies to every project. Our own single and double storey extension packages are quoted individually, with detailed estimates provided within 24 hours of enquiry through our home extension services.
Can I get planning permission changed after it’s been granted?
Yes, you can apply for a non-material amendment or a fresh application to vary an existing planning permission, depending on how significant the change is. Minor alterations are usually handled faster through a non-material amendment, while a more substantial redesign typically needs a new application assessed on its own merits.
Can I build a 3-metre rear extension without planning permission?
In most cases, yes: a 3-metre rear extension from the original wall falls within permitted development limits for houses that aren’t detached, provided height and eaves limits are also met. You should still confirm the property isn’t on designated land or subject to an Article 4 direction, since either removes this right even within the size limit, as explained in guidance on Article 4 restrictions.
Do I still need building regulations approval if my extension doesn’t need planning permission?
Yes. Permitted development only addresses whether you need planning permission; building regulations approval is a separate, mandatory process covering structural safety, fire protection and insulation standards, and it applies regardless of your planning route.
Sources
Before finalising any design, check the GOV.UK technical guidance, the Planning Portal’s extension guidance, and your local planning authority’s own design pages for Article 4 restrictions specific to your street.
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