Do You Need Planning Permission for a House Extension?

Key takeaways

• Many single-storey rear extensions are permitted development: up to 3 m deep on an attached house and 4 m on a detached house.
• Prior approval can extend that to 6 m (attached) or 8 m (detached), but neighbours are consulted.
• Conservation areas, flats, listed buildings and Article 4 directions usually mean a full planning application.
• A householder application costs £548 in England in 2026 and a Lawful Development Certificate £274; building regulations apply either way.

On this page (19 sections)

1. Quick answer: a six-question checklist
2. Two separate systems: planning and building regs
3. Permitted development limits for extensions
4. The original house rule and previous extensions
5. Larger home extensions and prior approval
6. When you need a full householder application
7. Common reasons extensions are refused
8. Lawful development certificate: why get one even under PD
9. Building regulations: always in the picture
10. The Party Wall Act
11. What happens if you build without permission
12. Planning permission in Southampton and Hampshire
13. How we handle it for you
14. How to check what has already been built
15. Is pre-application advice worth it?
16. What drawings you need and who prepares them
17. Trees, covenants and other checks
18. If your application is refused
19. Discharging conditions

Often, no. In England many single-storey rear extensions are permitted development, so they do not need planning permission if they stay within limits such as 3 metres deep on an attached house or 4 metres on a detached house and no more than 4 metres high. Larger extensions, two-storey extensions near a boundary, and any extension in a conservation area or on a flat or listed building usually do need a householder planning application, which costs £548 in 2026.

This guide covers how it works in England in 2026: the exact permitted development limits, the rules that catch people out, prior approval, fees and timescales, common reasons for refusal, what happens if you build without consent, and how it plays out locally in Southampton and Hampshire. Wales and Scotland have their own rules, and the larger home extension scheme described below applies in England only.

Quick answer: a six-question checklist

You will almost certainly need a full planning application if you answer yes to any of these:

1. Is the property a flat, maisonette or listed building?
2. Is it in a conservation area, national park, Area of Outstanding Natural Beauty or covered by an Article 4 direction?
3. Will it be two storeys and either more than 3 m deep, within 7 m of the rear boundary or in a conservation area, or a single storey extending more than 3 m (attached) or 4 m (detached) without using prior approval?
4. Will it extend forward of the front of the house, or to the side by more than half the width of the original house?
5. Will a single-storey extension be higher than 4 m, or have eaves above 3 m within 2 m of a boundary?
6. Will extensions and outbuildings together cover more than half the garden?

If you answered no to all six, your extension is probably permitted development. Read on to confirm, because the detail matters.

Two separate systems: planning and building regs

The single biggest source of confusion is that planning permission and building regulations are two completely different things, and most extensions need to satisfy both:

• Planning permission controls whether and what you can build: size, position, appearance, impact on neighbours.
• Building regulations control how it is built: foundations, structure, insulation, fire safety, drainage, ventilation, electrics.

You can have permitted development rights (no planning application needed) and still absolutely need building control sign-off. The two are not interchangeable.

Permitted development limits for extensions

Under permitted development, a single-storey rear extension can go 3 m on an attached house or 4 m on a detached house, up to 4 m high, and a side extension up to half the width of the original house.

Permitted development (PD) is a national grant of planning permission for certain works, as long as you stay within set limits. If your extension qualifies, you do not need to submit a full planning application. The rules apply to houses only, not flats, and they apply to the original house as it stood on 1 July 1948 or when it was built, if later. Previous extensions count against the limits.

Single-storey rear extensions

The most common PD project. Stay within 3 m (attached) or 4 m (detached), 4 m high, with eaves no higher than 3 m if within 2 m of a boundary, and you are usually clear. Flat roofs are fine within the height limits.

Side extensions

Single storey only, no more than half the width of the original house, no more than 4 m high. A side extension on a semi-detached house that reaches the boundary needs a party wall agreement even if it needs no planning permission.

Wrap-around extensions

An extension that goes beyond both the rear wall and a side wall must meet both sets of limits at once, cannot use the larger home extension scheme, and on article 2(3) land the side element is not permitted at all. In practice most wrap-arounds need a full householder application.

Two-storey extensions

More tightly controlled: no more than 3 m to the rear, at least 7 m from the rear boundary, roof pitch to match the existing house, and upper-floor side windows obscure-glazed. Many double-storey projects end up needing full planning permission.

Loft conversions, porches and outbuildings

Loft conversions have their own volume limit (see loft conversion costs and rules): 40 cubic metres on a terraced house and 50 on a semi or detached, with no extension beyond the front roof slope. Porches are permitted up to 3 square metres in floor area and 3 m high if at least 2 m from a highway. Outbuildings such as garden rooms are permitted up to 2.5 m high near a boundary, single storey, and must be incidental to the house, so a self-contained annexe to live in needs planning permission.

The original house rule and previous extensions

PD limits are measured from the original house: the house as first built, or as it stood on 1 July 1948. If a previous owner already added a rear extension, that depth counts. A 1930s semi with an existing 2 m rear extension has only 1 m of standard PD left, or 4 m under prior approval. Councils check historic aerial photos and old applications, so find out what has already been done before assuming you have the full allowance.

Larger home extensions and prior approval

The larger home extension scheme lets a single-storey rear extension go up to 6 m on an attached house or 8 m on a detached one without a full application. It is not automatic. You must notify the council with a prior approval application (£249), the council writes to adjoining neighbours and gives them 21 days to comment, and the council has 42 days to decide. If a neighbour objects, the council assesses the impact on their amenity and can refuse. If you hear nothing by day 42, approval is deemed granted, but confirm in writing before you dig. Other PD conditions, on height, eaves and materials, still apply.

When you need a full householder application

If the project falls outside PD, you submit a householder planning application to your local council. You will need:

• Scaled existing and proposed plans and elevations
• A location plan and block plan
• A short heritage statement where the council's local validation list asks for one in a conservation area
• The application fee: £548 in England in 2026, plus a Planning Portal service charge of around £90 for online submission

Fees now rise each April in line with inflation, so check the current figure before you apply. The council has 8 weeks to decide once the application is validated, and can ask for an extension of time if it needs to. Many councils offer pre-application advice for a modest fee, which is worth it for anything in a conservation area or near a difficult boundary.

Common reasons extensions are refused

Most refusals come down to a short list:

• Overlooking and loss of privacy from upper-floor windows or a raised terrace
• Loss of light to a neighbour, often tested with the 45-degree rule from their nearest window
• Overbearing impact: a two-storey extension close to a neighbour's boundary
• Overdevelopment: too much of the plot built on, or too little garden left
• Design and materials that do not respect the existing house or street, especially in conservation areas
• Loss of parking where a garage is converted or a driveway built over

A well-prepared application with good drawings addresses these before they become objections, which is why the design stage matters as much as the build.

Lawful development certificate: why get one even under PD

If your extension is permitted development, a Lawful Development Certificate (£274) is formal confirmation from the council that the work is lawful. It is not compulsory, but it removes doubt during the build, protects you if a neighbour complains, and is exactly what a buyer's solicitor asks for when you sell. Without it you may have to prove PD compliance years later from photographs and drawings. We recommend one on every PD extension.

Building regulations: always in the picture

Whichever planning route applies, your extension will need to meet current building regulations, with building control inspecting at key stages from foundations to completion. The key parts for an extension are:

• Part A structure, including foundations and any steel beams designed by an engineer
• Part L energy efficiency: insulation, glazing U-values and the overall performance of the extension
• Part O overheating applies to new homes only, but a heavily glazed south-facing extension should borrow its thinking on solar-control glass and shading
• Part F ventilation, and Part H drainage, including build-over agreements where the extension crosses a public sewer
• Part P electrical safety, certified by a registered electrician

You can apply with full plans, which are checked before work starts, or a building notice, where the inspector checks on site as you go. Fees run £600 to £1,200 for a typical extension. This is what confirms the extension has been built to the required standard for structure, insulation and safety, and you receive a completion certificate at handover, which buyers and their solicitors will ask for when you sell.

The Party Wall Act

Separate from both planning and building regs, the Party Wall etc. Act 1996 applies if you build on or near a shared boundary, which is common on semi-detached and terraced homes. You must serve notice on affected neighbours, two months ahead for work to a party wall and one month for excavation within 3 m of their foundations. If they consent in writing, you proceed; if not, surveyors are appointed and an award is drawn up. Budget £700 to £1,800 per neighbour with one agreed surveyor, or £1,500 to £3,500 if each side appoints their own. It is a legal process, not a planning one, and it never stops a lawful extension, but it does need to start early.

What happens if you build without permission

If an extension needed permission and did not get it, the council can serve an enforcement notice requiring you to alter or remove the work. You can apply for retrospective permission, which is judged as if it were a normal application, and appeal if it is refused. Since April 2024, unauthorised development in England only becomes immune from enforcement after ten years, up from four. Selling a house with an unconsented extension is difficult, because buyers' solicitors will ask for the consents and completion certificate and may demand indemnity insurance or a price reduction.

Planning permission in Southampton and Hampshire

Southampton City Council decides applications in the city, while Fareham, Eastleigh, Winchester, Test Valley and New Forest councils cover the surrounding towns, and each applies the rules a little differently. Around it, Fareham Borough Council has thirteen conservation areas including Titchfield with its Article 4 direction; Eastleigh Borough Council covers Hedge End and Chandler's Ford and publishes character area guidance planners refer to; Winchester City Council has thirty-seven conservation areas and thousands of listed buildings, with parts of the district decided by the South Downs National Park Authority; Test Valley Borough Council covers Romsey and its town-centre conservation area; and in Totton the New Forest National Park boundary west of the A326 changes the planning authority altogether. Tree preservation orders and Southern Water build-over agreements are the other local checks we make early.

How we handle it for you

We handle the planning route for you, so you do not have to decode it alone. At the free site visit we tell you honestly which route your extension falls under, before any money changes hands. If it is permitted development, we recommend applying for a Lawful Development Certificate to put it beyond doubt. If prior approval or full planning is needed, we manage the application end to end, prepare the drawings, and coordinate the structural engineer, party wall process and building control throughout the extension build.

Once you know your route, the next question is usually budget. Our guide to how much a house extension costs breaks down the numbers, including every fee mentioned here.

How to check what has already been built

Before you assume you have the full permitted development allowance, check the house's history. Your council's online planning portal lists every application on the address, going back decades in most areas. Historic aerial photographs, the estate agent's particulars from when you bought, and the deeds all help establish what was original. If a previous extension was built without consent, it may already have used up the PD allowance, and if it is more than ten years old (four years for work completed before 25 April 2024) it is probably immune from enforcement but still counts against the limits.

Is pre-application advice worth it?

For a straightforward rear extension on an ordinary house, no. For anything in a conservation area, near a difficult boundary, on a corner plot or involving a two-storey build, yes. Most councils offer written pre-application advice for a modest fee, and it tells you what the case officer will and will not accept before you spend money on a full set of drawings. It also starts the relationship with the officer who will decide the application.

What drawings you need and who prepares them

A householder application needs scaled existing and proposed floor plans, elevations and sections, usually at 1:50 or 1:100, plus a location plan at 1:1250 with the site outlined in red and a block plan at 1:500. An architect, architectural technologist or experienced designer prepares them, typically for £800 to £2,500 for a domestic extension, and the same drawings are the basis for the building regulations application and the builder's quote, so they are money well spent even where no planning application is needed.

Trees, covenants and other checks

A tree preservation order on a tree in your garden or a neighbour's means you need the council's consent for works to it, and its roots can dictate foundation depth. Trees in conservation areas are protected by default. Restrictive covenants in the deeds, common on newer estates, can require the developer's or a management company's consent for extensions regardless of planning. Rights of way and easements for drains or cables can block a footprint entirely. Your solicitor's report from the purchase usually lists these, and we check them at the site visit.

If your application is refused

A refusal is not the end. Read the officer's report, which sets out exactly why. A revised scheme can be resubmitted, but the free resubmission that used to apply was withdrawn in December 2023, so a second application carries the full £548 fee. Alternatively you can appeal to the Planning Inspectorate, which for a householder application must be lodged within twelve weeks of the decision and is decided on written representations, typically within a few months. Appeals succeed when the refusal misapplied policy, not simply because you disagree with it, so professional advice on which route to take is worth having.

Discharging conditions

Most permissions carry conditions, for example that materials be agreed before work starts or that the extension be built exactly to the approved drawings. Some conditions must be formally discharged by a separate application with a small fee before you begin. Starting work without discharging a pre-commencement condition can invalidate the permission, so read the decision notice carefully or ask us to.

Planning rules change and every property is different, especially in conservation areas, so treat this as a guide rather than the final word. Figures are correct for England as of August 2026. The safest first step is a site visit where we can look at your specific home and give you a clear answer.

Written by

The Pure Renovation & Design team

A Southampton design-and-build company whose team has 18 years of hands-on experience in extensions, loft conversions, full renovations, bathrooms and joinery across Hampshire. Prices are cross-checked against published UK trade cost guides and official fee schedules, and reviewed whenever prices or rules change.

Last reviewed 13 October 2026 · About the team

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