3m/4m and 6m/8m Limits: Extension Depth Rules for England Homeowners

Most single-storey rear extensions in England can extend 3 metres beyond the original rear wall on terraced or semi-detached houses, or 4 metres on detached houses, without needing full planning permission. Go beyond that, up to 6 metres or 8 metres, and you enter the prior approval route, which requires neighbour consultation. Two-storey and side extensions follow separate rules entirely, and designated land can remove these rights altogether. Always measure from the original rear wall and check with Planning Portal or your local council before you commit to a design.

TL;DR:

• Extensions on terraced or semi-detached houses can generally stay within 3 meters; detached houses have a 4-meter limit unless they qualify for the larger scheme.
• The total depth of an extension must be measured from the original rear wall as of 1 July 1948; previous enlargements like conservatories count towards the limit.
• Extensions exceeding 6 meters on terraced or semi-detached houses or 8 meters on detached homes require full planning permission, not just permitted development or prior approval.
• Two-storey and side extensions cannot use the prior approval route and have stricter siting and privacy restrictions, especially on designated land.
• Accurate measurement and early consultation with local authorities, surveys, and legal advice are essential to avoid planning enforcement issues and ensure compliance.

Table of Contents

• At-a-glance permitted development and prior approval depth limits
• How to measure extension depth from the original rear wall
• The larger home extension route: prior approval explained
• Two-storey and side extensions: different rules apply
• Building regulations, foundations and structural design
• Pre-start checklist and when to get a lawful development certificate
• How extension depth affects property value and resale
• Combining depth rules with conservation areas and listed buildings
• Common reasons extension depth proposals get refused
• Why homeowners get extension depth wrong
• How The Extension Works handles permitted development and prior approval
• Sources
• FAQ

At-a-glance permitted development and prior approval depth limits

Before you sketch anything, it helps to see the actual numbers side by side. These are the depth limits that decide whether your project is permitted development, needs prior approval, or requires a full planning application.

These figures come from the Gov, which sets both the standard limits and the thresholds for the larger home extension scheme.

A few points worth clarifying before you take a tape measure to your garden:

• The “detached” distinction matters: semi-detached and terraced houses get the lower 3 metre standard limit and 6 metre prior approval ceiling.
• Anything beyond 6m or 8m falls outside permitted development entirely and needs a standard planning application.
• Two-storey and side extensions cannot use the prior approval route at all, regardless of how modest the depth is.
• Article 2(3) designated land, covered later, can strip these permitted development rights away completely.

How to measure extension depth from the original rear wall

The “original rear wall” is the wall as it stood on 1 July 1948, or as built if the house was constructed after that date. This matters because many houses already carry an extension, conservatory or garage conversion, and the rules do not let you treat each new addition as a fresh 3 metre or 4 metre allowance.

Measurements are cumulative. The GOV.UK guidance confirms that permitted development limits apply to the total enlargement joined to the original dwelling, so a homeowner cannot add a further projection that pushes the combined depth past the 3m/4m standard cap or the 6m/8m prior approval threshold.

A few practical pitfalls catch people out:

• A 1980s conservatory bolted onto the rear wall still counts towards your total depth allowance, even if it looks temporary.
• Changes in ground level can make a single-storey extension read as taller than expected when measured against eaves height rules.
• Joined extensions from different decades are added together as one continuous projection, not assessed separately.
• A previous side return infill can eat into the width allowance available for a further side extension.

If your house has been extended before, get the original plans or a measured survey before assuming you have the full 3m or 4m still available.

The larger home extension route: prior approval explained

Once your proposed depth exceeds the standard PD limit, you move into what the guidance calls the larger home extension scheme, more commonly known as prior approval or the neighbour consultation scheme. It only applies to single-storey rear extensions, it is not available on designated land, and it covers depths between 3m and 6m (other houses) or 4m and 8m (detached houses), as set out in the GOV.UK technical guidance.

The process runs roughly like this:

1. Submit a notification to your local planning authority describing the proposed extension, with plans and the addresses of adjoining properties, following the format set out in Planning Portal’s prior approval guidance.
2. The council writes to adjoining neighbours, who then have a 21 day window to raise objections.
3. If no objections arrive, or the council fails to issue a decision within its 42 day window, the extension can typically proceed under the notified terms.
4. If objections are raised, a planning officer assesses the scheme against neighbour amenity, considering loss of light, overshadowing and visual impact before deciding whether to refuse or allow it.

Councils are not assessing design quality here, only the effect on neighbouring amenity. Objections most often relate to loss of light or a sense of enclosure rather than the extension’s appearance.

Pro Tip: Send neighbours an informal sketch and a friendly conversation before the council notification lands on their doormat. It rarely stops an objection outright, but it often turns a hostile one into a measured one.

Two-storey and side extensions: different rules apply

Depth limits for two-storey work and side extensions are stricter than the single-storey rules, and they cannot use the prior approval route at all.

• A two-storey rear extension is usually limited to 3 metres beyond the original rear wall, and it must sit at least 7 metres from the boundary opposite the rear wall.
• Upper-floor side windows facing a boundary typically need to be obscure-glazed and non-opening below 1.7 metres, to protect neighbour privacy.
• Side extensions are restricted to single-storey only under permitted development, with a maximum width of half that of the original house.
• Local guidance often applies a “terracing” test: where a gap between semi-detached or terraced houses is a key part of the street’s character, filling it can be refused even if it technically meets the numeric limits, a concern flagged in the Crawley householder development SPD.
• On Article 2(3) designated land, which includes conservation areas, National Parks, Areas of Outstanding Natural Beauty and World Heritage Sites, several permitted development rights are removed or reduced, meaning many extensions that would otherwise be automatic need a full planning application.

If your street has a run of identical semi-detached houses with narrow side gaps, assume terracing guidance will apply and check your local SPD before designing a side extension that fills the gap entirely.

Building regulations, foundations and structural design

Planning permission and Building Regulations are two separate approvals, and meeting one does not exempt you from the other. Even a permitted development extension needs to satisfy Building Regulations covering structural stability, drainage, fire safety and thermal efficiency.

Foundation depth is where this becomes technical fastest. A single-storey rear extension on stable ground might need only a standard strip foundation, but a two-storey addition, a build on clay or sloping ground, or anything involving significant excavation usually needs structural calculations and a formal Building Regulations application. This is also where drainage runs, party wall considerations and steel beam sizing get decided, none of which planning permission touches.

Pro Tip: Bring a structural engineer in at the sketch stage, not after the planning drawings are finished. Foundation and beam decisions can quietly change your layout, and it is far cheaper to adjust a drawing than to dig twice.

Our guide to structural calculations for extensions and our foundations guide cover the technical detail if you want to understand what a Building Control officer will be checking.

Pre-start checklist and when to get a lawful development certificate

Before you order materials or book a contractor, run through a short checklist to catch the mistakes that cause enforcement problems later.

• Confirm whether your property sits on Article 2(3) designated land, which removes or restricts permitted development rights.
• Measure your proposed depth from the true original rear wall, not from an existing extension.
• Add up any previous enlargements to check your total does not exceed the standard or prior approval caps.
• Check your local authority’s supplementary planning document for design or amenity tests that go beyond the national rules.
• Read Planning Portal’s prior approval guidance if your depth sits above the standard PD limit.

A Lawful Development Certificate is worth the modest cost whenever a scheme is borderline, whenever you plan to sell within a few years, or whenever your property sits near designated land boundaries. It gives you a formal council decision confirming the work was lawful, which solicitors and buyers will ask for during conveyancing. Our guide to lawful development certificates covers the application process in more detail.

How extension depth affects property value and resale

A well-proportioned rear extension that stays within permitted development limits tends to add usable space without creating a paper trail of planning risk. The value lift comes from the extra square metreage and improved layout, typically an enlarged kitchen-diner, rather than from depth alone. Buyers and their solicitors care less about how many metres you built and more about whether the work was lawful.

This is where depth rules and resale value intersect directly. An extension built without the right consent, or one that quietly drifted past its permitted depth during construction, becomes a conveyancing problem. Solicitors acting for a buyer will ask for evidence that the extension was either permitted development or granted planning permission, and a missing Lawful Development Certificate on a borderline scheme can slow or derail a sale.

Overbuilding also carries a design risk that affects value in a different way: a deep single-storey extension that swallows too much garden or overshadows a kitchen can make a house harder to sell, even when the paperwork is faultless. The 50% curtilage rule referenced by Planning Portal, which caps the proportion of your garden that can be covered by buildings, exists partly to prevent this. A sensible approach treats the legal maximum as a ceiling to consider, not a target to hit by default.

Combining depth rules with conservation areas and listed buildings

Extension depth rules do not operate in isolation. Conservation area status, listed building consent and Article 2(3) designations can all sit on top of the national permitted development limits, and any one of them can remove your automatic rights regardless of how modest your proposed depth is.

If your house is in a conservation area, several permitted development rights are restricted, including some extension types that would be automatic elsewhere. Listed buildings need listed building consent for almost any alteration, inside or out, irrespective of depth. In both cases, the safest starting point is your local authority’s conservation officer or planning duty desk, not the national PD limits alone.

Practical steps that help:

• Check your address against your council’s conservation area map before finalising any depth measurement.
• If listed, speak to the council’s conservation team early. Depth becomes a secondary question once listed building consent is needed.
• Expect stricter material and design scrutiny in conservation areas even when the numeric depth is within normal PD limits.
• Local SPDs, such as Crawley’s householder development guidance, often apply neighbour amenity tests using a 45 or 30 degree assessment from neighbouring windows, which can defeat a scheme that meets the national depth limit but harms daylight to an adjoining property.

Never assume a compliant depth measurement is the end of the conversation. Designation status is checked first, because it decides which rulebook applies at all.

Common reasons extension depth proposals get refused

Most refusals are not really about the headline depth figure. They come from how that depth interacts with the site.

Loss of light and outlook to neighbouring windows is the most frequent objection raised during prior approval consultation, particularly where a deep single-storey extension sits close to a shared boundary. Overshadowing of a neighbour’s garden or main window follows closely behind, especially with extensions that push towards the 6m or 8m upper limits.

Terracing effect refusals occur where filling a gap between semi-detached or terraced properties damages the rhythm of a street, a concern several local SPDs address explicitly. Exceeding the 50% curtilage coverage limit is a straightforward technical refusal that catches homeowners who forget to add up existing outbuildings and previous extensions. And schemes on designated land that proceed as if permitted development still applied, without checking Article 2(3) status first, are refused simply because the right never existed for that plot.

The pattern across all of these is the same: depth measured correctly still fails when it is not paired with an honest look at the site’s constraints and its neighbours’ outlook.

Why homeowners get extension depth wrong

The most common mistake we see is measuring from the wrong wall, usually an existing extension rather than the true original rear wall, which understates how much of the allowance has already been used. A close second is assuming a conservatory built decades ago does not count towards the total, when it almost always does.

The process ideally starts with an accurate measured survey and a check against the property’s planning history before any design work begins, then builds the 3D model and paperwork around what is actually achievable. That sequencing is what prevents a homeowner from falling in love with a depth the site cannot legally support.

— Esskay

How The Extension Works handles permitted development and prior approval

Working out whether your extension needs prior approval, a full planning application, or nothing at all is the part of the process most homeowners find hardest, and it is exactly where a design-and-build service earns its keep. The entire extension process can be managed in-house: measurement, planning strategy, prior approval notifications where needed, and construction, without homeowners having to coordinate an architect, a planning consultant and a builder separately.

• Get an instant online quote and see your extension modelled in live 3D before committing.
• Planning and prior approval paperwork, including neighbour notifications, can be handled as part of services.
• Pricing may be fixed and VAT-inclusive, agreed before work starts, with a workmanship warranty on completion.

Start with an instant quote for your single-storey rear extension to see a fixed price and a 3D model of your project before you book.

This article is general information, not a substitute for advice from a qualified lawyer. Consult a qualified legal professional about your own circumstances before acting on anything here.

Sources

• Gov
• Planning Portal — extensions planning permission
• Crawley householder development supplementary planning document (May 2026)

FAQ

What is the 3 metre rule for extensions?

The 3 metre rule refers to the standard permitted development limit for single-storey rear extensions on terraced and semi-detached houses in England, measured from the original rear wall. Detached houses get a slightly larger 4 metre allowance under the same GOV.UK guidance.

Can I extend more than 6 metres?

Yes, but only via the prior approval route for single-storey rear extensions, which allows up to 6 metres on terraced and semi-detached houses or 8 metres on detached houses, subject to neighbour consultation under the GOV.UK technical guidance. Beyond those figures, or for two-storey and side extensions, you need a full planning application instead.

How deep should footings be for a two-storey extension?

Footing depth depends on ground conditions, soil type and the load of the structure above, so it is set by a structural engineer rather than a fixed national figure. Building Regulations, not planning permission, govern this, and early input from a structural engineer or Building Control is the recommended route.

What are the general rules for extensions in England?

Single-storey rear extensions typically follow the 3m/4m standard limits or the 6m/8m prior approval thresholds, while two-storey and side extensions face stricter siting and privacy rules, as set out on Planning Portal. Designated land, including conservation areas, can remove these permitted development rights entirely, so checking your property’s status is the first step.

Recommended

• Prevent Planning Delays: England’s 4m and 3m Extension Height Limits
• Planning fees for an extension: what England homeowners need to know
• How big can a rear extension be under permitted development?
• Foundation depth for extension: what you need to know