Planning, permitted development and Party Wall before you extend in London: what needs permission and how to brief a builder

Most London extension disputes begin months before the first foundation is dug: with an assumption that "it's permitted development", a neighbour who only hears about the steel when the skip arrives, or a builder priced without the drawings a conservation officer will demand. Getting the consent path straight is how you protect both the budget and the relationship next door.

This guide walks through when householder permitted development can apply, when you need full planning permission or prior approval, how the Party Wall etc Act 1996 fits alongside, what fees look like from April 2026, and how to brief a builder so quotes are comparable. It reflects official guidance checked on 19 October 2026. It is not a substitute for advice from your local planning authority or a qualified party wall surveyor.

Permitted development is a rulebook, not a vibe

Householder permitted development rights in England are set out in the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended. The Ministry of Housing's technical guidance stresses that a proposal must meet all the limits and conditions of the relevant Class - and that those rights do not remove the need for building regulations approval or for consents under the Party Wall Act.

For a typical single-storey rear extension under Class A, the guidance explains that the enlarged part must not extend beyond the rear wall of the original dwellinghouse by more than 4 metres if detached, or 3 metres in any other case, and must not exceed 4 metres in height. "Original" means the house as first built, or as it stood on 1 July 1948 if built earlier - previous extensions count against you.

A separate provision allows larger single-storey rear extensions - up to 8 metres (detached) or 6 metres (other houses) - on homes that are not on article 2(3) land (which includes conservation areas) and not on a site of special scientific interest, but only through the neighbour-consultation prior-approval process. Height remains capped at 4 metres for that route. Side extensions, eaves height, width relative to the original house, materials of similar appearance, and treatments of principal elevations each have their own tests in the same guidance.

When London homes usually need a full application instead

• Flats and maisonettes - householder permitted development rights for extensions do not apply in the same way; expect to need planning permission.
• Listed buildings - listed building consent may be required in addition to any planning permission; PD does not override listing controls.
• Conservation areas and article 2(3) land - certain PD rights are narrowed or removed; the larger rear-extension PD route is not available on article 2(3) land.
• Article 4 Directions - many London boroughs have removed selected PD rights in specific streets or estates. Always check your borough's Article 4 map before assuming PD applies.
• Two-storey rear extensions, wraparounds that fail width/height tests, front extensions, and work beyond PD limits - these commonly need a householder planning application.

If you want certainty that a PD scheme is lawful, apply for a Lawful Development Certificate for the proposed development. It is optional but often wise before you spend on steel.

Larger rear extensions: the prior-approval sequence

Where you rely on the larger single-storey rear PD right, you must notify the local planning authority, which then consults adjoining owners. As boroughs such as Merton summarise from the national rules: work must not start until the authority has written that prior approval is not required, has given approval, or 42 days have passed without a decision. If a neighbour objects, the authority assesses the impact on amenity. The development must still meet every other Class A limit - prior approval is not a free pass on height, materials or side-wrap rules.

Fees you should budget (England, from 1 April 2026)

According to the Planning Portal schedule of fees applying from 1 April 2026:

• Householder application to enlarge, improve or alter an existing single dwellinghouse (excluding flats): £548
• Prior approval - larger home extensions (Part 1, Class A): £249
• Lawful Development Certificate (proposed): half the equivalent full application fee
• Lawful Development Certificate (existing): same as the full application fee

Architect, structural engineer and party wall surveyor fees sit on top. HomeOwners Alliance's extension costs guide cites party wall agreements averaging around £1,000, varying with complexity and the number of surveyors involved.

Party Wall: the notice your neighbour is entitled to

GOV.UK's party wall guidance requires you to tell adjoining owners if you plan building work on or near a shared property boundary or party wall in England and Wales. That includes works to an existing party wall (cutting in steels, raising, underpinning), new walls on the boundary, and certain excavations near neighbouring foundations - the detail is in the Party Wall etc Act 1996 explanatory booklet.

GOV.UK advises giving written notice between two months and a year before you plan to start, describing the works. You may speak to neighbours first, but agreements should be in writing. If they dissent or do not reply within the statutory period, surveyors are appointed and an award sets out how the work may proceed. Planning permission does not replace this process; building regulations approval does not either.

For terraced and semi-detached London houses, party wall notices are the norm rather than the exception for rear and side extensions that bear on shared walls or dig near them. Build the notice period into your programme before you promise a start date on site.

Building regulations are still mandatory

Even a PD extension must comply with building regulations - structure, fire, insulation, ventilation, electrics, drainage and more. GOV.UK explains when building regulations approval is needed. Keep the completion certificate: buyers' solicitors will ask for it.

How to brief a builder once the consent path is clear

Do not ask builders to "price an extension" from a sketch alone if you want comparable tenders. A brief that produces useful quotes usually includes:

1. existing and proposed plans, elevations and a simple specification;
2. confirmation of the consent route - PD with or without prior approval, householder permission granted, or application pending;
3. structural engineer's drawings where steels or foundations are non-standard;
4. known constraints - conservation-area materials, tree roots, shared drains, limited side access;
5. whether the price should include party wall surveyor attendance, scaffolding, skip permits and parking suspensions;
6. kitchen and bathroom - included, excluded, or provisional sums;
7. a request for a staged payment schedule linked to inspections.

Vet whoever you shortlist: match limited-company details on Companies House, confirm TrustMark or trade-body claims on official registers, and keep deposits within the bounds Citizens Advice recommends (as low as practical, not more than 25 per cent, with protection where possible). The Consumer Rights Act 2015 still requires reasonable care and skill once they start.

A sensible order of operations

1. Measure what you have and decide the outcomes you need (light, table space, accessible bathroom) before fixing a glamorous footprint.
2. Check PD, Article 4, listing and conservation status with your borough or a planning professional.
3. Appoint design and structural input appropriate to the risk.
4. Submit the right planning or prior-approval forms - or an LDC if relying on PD.
5. Serve Party Wall notices in good time.
6. Tender to builders on a fixed information pack.
7. Sign a written contract with staged payments and insurance checked.
8. Do not let demolition start until the consent and notice position is clear.

London rewards the homeowner who treats planning and neighbours as part of the build, not an afterthought. A builder who asks about your Party Wall status before offering a start date is usually the same one who will still be answering the phone at snagging.

Sources checked on 19 October 2026: GOV.UK, Permitted development rights for householders: technical guidance; GOV.UK, When is permission required?; GOV.UK, Party walls and building work; GOV.UK, Party walls: when and how to tell them; Party Wall etc Act 1996 explanatory booklet; Planning Portal, Fees for planning applications in England (from 1 April 2026); HomeOwners Alliance, House extension costs; GOV.UK, Building regulations approval; Consumer Rights Act 2015, s.49. Nothing here is legal or planning advice; confirm with your local planning authority.