Do I Need Planning Permission for My Extension?
Whether you need planning permission for a house extension in England depends on whether the works fall within the limits of permitted development set out in the General Permitted Development Order 2015, Schedule 2, Part 1, Class A. Many single-storey rear extensions, some side extensions, and most loft conversions can be built without a full planning application, provided they stay within specific height, depth, and coverage limits and the property is not a flat, is not listed, and has no Article 4 Direction removing that right. Step outside those limits — or the property falls into one of the excluded categories — and a full householder planning application is required instead, assessed against the borough's local plan under the Town and Country Planning Act 1990.
Because the rules turn on precise measurements and site-specific exclusions, the safest way to confirm a scheme's status before building is a Lawful Development Certificate under section 192 of the 1990 Act, a formal, legally reliable answer rather than a guess based on general rules of thumb.
What size extension can you build without permission?
On a terrace or semi, 3m deep as of right and 6m under prior approval; on a detached house, 4m and 8m. Height is capped at 4m, or 3m at the eaves within two metres of a boundary, and all extensions together may cover no more than half the curtilage.
| Limit | Terrace or semi | Detached |
|---|---|---|
| Single-storey rear depth, as of right | 3m | 4m |
| Single-storey rear depth, prior approval | 6m | 8m |
| Maximum height, single storey rear | 4m | 4m |
| Eaves height within 2m of a boundary | 3m | 3m |
| Side extension width | half the original house | half the original house |
| Ground covered by extensions and outbuildings | 50% of the curtilage | 50% of the curtilage |
| Any of the above on Article 2(3) land | not permitted development | not permitted development |
Source: Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A, as amended, read at 1 September 2026. ‘Original house’ means the house as first built or as it stood on 1 July 1948, not as you bought it — a previous owner’s extension counts against your allowance. Article 2(3) land covers conservation areas, National Parks, the Broads, AONBs and World Heritage Sites.
What Size Extension Can I Build Without Planning Permission?
Under GPDO 2015 Schedule 2, Part 1, Class A, a single-storey rear extension to a house can generally be built under permitted development up to 4 metres deep for a detached house, or 3 metres deep for any other house (terraced, semi-detached), without a full planning application — provided it also does not exceed 4 metres in height, or 3 metres in height within 2 metres of a boundary. Under the temporary-turned-permanent Larger Home Extension neighbour consultation scheme, those depths can be extended further, to 8 metres for a detached house and 6 metres for other houses, but only via a prior approval application: the council must notify adjoining neighbours and can refuse if they object on grounds of impact, even though it is technically still a permitted development route rather than full planning permission.
Side extensions are more constrained: under Class A they are generally limited to a single storey, maximum 4 metres in height, and no wider than half the width of the original house. Two-storey rear extensions do not benefit from the larger single-storey allowances at all and are capped at 3 metres depth, again subject to eaves and ridge height limits relative to the existing roofline.
What Is the Larger Home Extension Scheme?
The Larger Home Extension scheme, formally the neighbour consultation scheme under the GPDO, allows single-storey rear extensions beyond the standard Class A depth limits — up to 8 metres for detached houses and 6 metres for others — without needing full planning permission. It is not automatic: the applicant submits a prior approval application, the council notifies adjoining neighbours, and if any neighbour objects within the consultation period, the council must consider the impact on their amenity and can refuse prior approval, at which point the extension would need a standard planning application instead.
If no neighbour objects within the statutory period, or the council decides any objection does not justify refusal, prior approval is granted and the extension can proceed as permitted development. This route sits between full planning permission and unconditional permitted development, and a chartered architectural technologist will usually recommend it only where the neighbour relationship is stable, since a single sustained objection can still block the larger scheme.
Does a Loft Conversion Need Planning Permission?
Many loft conversions in houses (not flats) fall within permitted development under GPDO Schedule 2, Part 1, Class B, subject to a volume allowance — generally up to 40 cubic metres of additional roof space for terraced houses and up to 50 cubic metres for detached and semi-detached houses. The conversion must not extend beyond the existing roof slope facing a highway, must use materials that are similar in appearance to the existing house (except for conservatory-type roof lights), and any side-facing windows must be obscure-glazed and, where less than 1.7 metres above the floor, non-opening.
A dormer window is usually permitted development within that volume allowance, but a mansard roof conversion — which alters the roof structure and pitch across a wider area — more often needs full planning permission, particularly on a terraced property where the change to the roofline is visible along the whole street and affects the building's overall form more significantly than a rear dormer.
When Does a Property Lose Permitted Development Rights?
- Flats and maisonettes — permitted development rights under GPDO Schedule 2 Part 1 apply to houses only; flats have no equivalent automatic allowance and virtually all alterations need a full planning application (or, for internal-only works, may not need consent at all, but any external change does).
- Article 4 Directions — a council can remove specific permitted development rights across a defined area under section 4 of the Town and Country Planning Act 1990, most often to protect conservation area character. Where an Article 4 Direction applies to a property, works that would otherwise be permitted development need a full planning application instead.
- Listed buildings — permitted development rights for external alteration are effectively suspended; almost any change, internal or external, needs separate listed building consent regardless of scale.
- New-build restrictions — some new developments have their permitted development rights removed by planning condition on the original permission, meaning even minor later alterations need a fresh application.
Article 4 Directions are common in parts of Camden, Islington, Richmond upon Thames, and several other boroughs with extensive conservation area coverage; checking the specific address against the council's adopted Article 4 map is a standard first step before assuming permitted development applies.
How Do I Prove My Extension Doesn't Need Planning Permission?
The formal route is a Lawful Development Certificate, applied for under section 192 of the Town and Country Planning Act 1990 for a proposed use or development, or section 191 for an existing one. The council assesses the application against the same permitted development rules a full planning application would be measured against, but the outcome is a certificate confirming lawfulness rather than a grant of permission, and it carries its own, separate fee, generally around half the equivalent planning application fee.
A Lawful Development Certificate is not compulsory, but it is strongly advisable before building anything relying on permitted development rights, because it removes any future doubt for mortgage lenders, conveyancing solicitors, and buyers when the property is eventually sold. Building without one, relying only on an informal view that the works are permitted development, leaves the burden of proof on the homeowner if the council or a future buyer's solicitor later disputes it.
How Does Permitted Development Interact With Building Regulations?
Permitted development status only answers the planning question — whether the works need planning permission — and has no bearing on whether they need building regulations approval, which is assessed separately under the Building Act 1984. An extension or loft conversion built entirely under permitted development still needs a full building regulations application or a building notice, covering structural design, thermal performance, means of escape, and drainage, before or during construction. Confusing the two is a common and costly mistake: homeowners sometimes assume that because no planning permission was needed, no consents at all were required, and then struggle to sell the property without a completion certificate for building regulations.
What About Change of Use?
Changing how a building or part of a building is used — converting a house into flats, turning a shop into a residential unit, or adding a self-contained annexe used independently from the main house — can require planning permission even where no physical building work is involved, because the Town and Country Planning Act 1990 treats a material change of use as development in its own right. Some changes of use are themselves covered by permitted development rights under separate GPDO classes (certain office-to-residential conversions, for example), but these are narrower and more frequently restricted by local Article 4 Directions than the extension and loft conversion classes, particularly in boroughs seeking to protect commercial floorspace or family housing stock.
Any change of use scheme should be checked against the specific GPDO class it might fall under, and against local Article 4 restrictions, before assuming permitted development applies — the exceptions here are more borough-specific than for straightforward extensions.
What About Outbuildings and Garden Rooms?
Outbuildings such as garden offices, garden rooms, and sheds are covered by their own permitted development class under GPDO Schedule 2, Part 1, Class E, separate from the Class A rules for extensions. Broadly, a single-storey outbuilding can be built under permitted development provided it is not forward of the principal elevation, covers no more than 50% of the total area of land around the original house once all outbuildings and extensions are combined, and, if within 2 metres of a boundary, does not exceed 2.5 metres in overall height. An outbuilding used as self-contained separate living accommodation, rather than incidental to the enjoyment of the house, falls outside these permitted development rights entirely and needs a full planning application, since that use is treated as a separate dwelling in planning terms.
Basement extensions under an existing garden, by contrast, are rarely covered by permitted development at all in practice, both because of their scale relative to Class A volume limits and because several boroughs, Kensington and Chelsea most prominently through its Basement SPD, apply specific policy tests that only a full planning application can properly address.
What Happens if I Build Without Checking First?
If work turns out to exceed permitted development limits or need planning permission that was never sought, the council can take enforcement action under the Town and Country Planning Act 1990, generally within four years for operational development (building work) or ten years for a change of use, though these periods vary by circumstance. In practice, many unauthorised works are resolved through a retrospective planning application rather than enforcement, but that application is assessed exactly as if the works had not yet started, with no guarantee of approval, and can be considerably more stressful and costly than getting the drawings and permitted development check right beforehand.
A chartered architectural technologist will check a scheme against the current GPDO limits, the property's Article 4 status, and its listing status as a first step, before any drawings are finalised, precisely to avoid this scenario.
Frequently asked questions
Can I build a single-storey rear extension without planning permission?
Often yes, under permitted development, provided it stays within 4 metres depth for a detached house or 3 metres for any other house, does not exceed 4 metres in height (3 metres within 2 metres of a boundary), and the property has no Article 4 Direction, is not a flat, and is not listed. A Lawful Development Certificate can formally confirm this before building.
What is the Larger Home Extension scheme?
It is the GPDO neighbour consultation scheme allowing single-storey rear extensions up to 8 metres deep (detached) or 6 metres (other houses) without full planning permission, subject to a prior approval process in which the council notifies neighbours, who can object and trigger a refusal of prior approval based on amenity impact.
Does a two-storey extension need planning permission?
It is more likely to. Two-storey rear extensions under permitted development are limited to 3 metres depth and must respect eaves and ridge height limits relative to the existing house; many two-storey schemes exceed these limits and need a full householder planning application.
What is an Article 4 Direction?
An Article 4 Direction is a mechanism under section 4 of the Town and Country Planning Act 1990 allowing a council to remove specific permitted development rights across a defined area, usually to protect conservation area character. Where it applies, works that would normally be permitted development need a full planning application.
Do flats have permitted development rights for extensions?
No. Permitted development rights under GPDO Schedule 2 Part 1 apply to houses, not flats or maisonettes. Virtually any external alteration to a flat, including extensions and loft conversions, needs a full planning application.
What is a Lawful Development Certificate and do I need one?
It is a formal certificate under section 192 (proposed works) or section 191 (existing works) of the Town and Country Planning Act 1990, confirming that specific works are lawful without planning permission. It is not compulsory but is strongly recommended, since it protects you against future disputes with buyers, lenders, or the council.
Can a mansard loft conversion be built under permitted development?
Rarely. A mansard conversion typically alters the roof pitch and structure across a wide area, changing the building's overall form more significantly than a dormer, which usually takes it outside the Class B permitted development volume and design limits and into full planning permission territory.
