London Rear Extension Guide — GPDO Class A, Prior Approval, Part Wall, £2,800-£4,500/m²

Your neighbour has 21 days to object once the Planning Officer writes to them. Here is what happens inside the 42-day Prior Approval window, which London boroughs have quietly switched those rights off, and what a 5-metre open span costs once the Structural Engineer has priced the steel.

Most London rear-extension advice is written by people who have never served a Party Wall notice. Permitted development under GPDO Class A is treated as a birthright, Prior Approval as a rubber-stamp, and the Neighbour Consultation Scheme as a formality. None of that is true in London. Between 20 and 30 per cent of inner-London postcodes sit under an Article 4 Direction that removes Class A rights entirely, the Party Wall etc. Act 1996 fires automatically on any rear extension within three metres of a neighbour's foundations, and the 5 to 7 metre open span that makes a kitchen-diner feel like a kitchen-diner needs signed calculations, a steel fabricator, and Building Control sign-off or the Completion Certificate never arrives.

What GPDO Class A actually allows (and doesn't)

The Town and Country Planning (General Permitted Development) (England) Order 2015 — GPDO 2015 — governs extensions without full planning. Schedule 2, Part 1, Class A is the most-used and most-misunderstood provision in the Order. Headline single-storey rear-extension limits:

• 3 metres depth beyond the original rear wall for an attached house (terrace or semi-detached)
• 4 metres depth for a detached house
• 4 metres maximum height, or 3 metres if within 2 metres of a boundary
• Eaves must not exceed existing eaves height
• Side extensions are single-storey only, max 4m height, no wider than half the original dwelling

The Larger Home Extension route doubles rear depth to 6 metres attached and 8 metres detached, but only via a Prior Approval application under the Neighbour Consultation Scheme. You do not have this right automatically — you apply, the Planning Officer notifies every adjoining rear-boundary owner, and it goes live only if the officer grants it or the 42-day deemed-consent clock runs out.

Class A is removed entirely if: the property is a flat or maisonette; the house is listed; it sits in a Conservation Area with Class A restrictions; it is in an AONB or National Park; or an Article 4 Direction removes the right. Class A is cumulative — previous extensions count against your allowance, and buying the house does not reset it.

The last two points are where London projects die. An architect describing a rear extension as "permitted development, no planning needed" in Tottenham, Spitalfields, Bow, or parts of De Beauvoir is wrong — those postcodes sit under Article 4 Directions in force for years. Run the borough's Local Plan map before design, not after.

Prior Approval and the Neighbour Consultation Scheme

The Larger Home Extension route runs via a Prior Approval application — not full planning, not a Lawful Development Certificate, but a statutory consultation procedure giving neighbours a formal objection window:

1. Homeowner submits application (drawings, site plan, dimensions) with a £120 fee.
2. Planning Officer notifies every adjoining owner sharing a rear boundary.
3. Neighbours have 21 calendar days to object in writing on amenity grounds (overshadowing, loss of light, dominance), not general planning matters.
4. If no objection, Prior Approval can issue at any point during the remaining window.
5. If objections are received, the officer weighs them on their merits and may refuse.
6. The total determination window is 42 calendar days. If the officer has not decided by then, Prior Approval is deemed granted.

Two realities London architects understate: objections are standard, not rare — in Camden, Hackney, and Tower Hamlets, 35-50 per cent of Larger Home Extension applications attract at least one. And refusal is a real outcome — the only appeal route is a full planning application at 8-13 weeks on top of the 42 days already burned.

Article 4 Directions by London borough

An Article 4 Direction is a legal instrument under Article 4 of the GPDO that removes specific permitted development rights across a defined geographic area. Inside the zone, the removed right ceases to exist and any work that would have used it now requires full planning. London boroughs with Article 4s currently affecting rear extensions (non-exhaustive — always verify on the borough map):

• Hackney — De Beauvoir, Stoke Newington, Clapton, Dalston, London Fields conservation areas
• Camden — most of the borough's conservation areas including Primrose Hill, Fitzrovia, Hampstead, Bloomsbury
• Kensington and Chelsea — most conservation areas, which is most of the borough
• Haringey — Tottenham — borough-wide Article 4 adopted in 2019
• Tower Hamlets — Spitalfields, Bow, Whitechapel — multiple Article 4s across conservation areas
• Islington — most conservation areas, which is most of the borough
• Southwark — Bermondsey, Dulwich Village, Herne Hill
• Lambeth — Brixton, Clapham Old Town, several conservation areas
• Wandsworth — Battersea, Clapham Common, Balham

Run the borough's constraints map and Local Plan layer for Article 4 coverage at your exact address before design. Every borough publishes a free interactive map. Ignoring it is the most common reason London projects restart as full planning applications six weeks after the deposit is paid.

When full planning permission is required

Full planning is the default, not the exception. Required when: the extension exceeds Class A limits without qualifying for Larger Home Extension Prior Approval; the property is in a Conservation Area beyond the reduced Class A allowance; it is listed; it sits under an Article 4; it is in an AONB; it proposes a two-storey extension beyond Class AA limits; or cumulative works exceed the Class A allowance.

GPDO Schedule 2 Part 1 Class AA (added 2020) permits upward extensions: up to one additional storey on an originally single-storey dwellinghouse; up to two additional storeys on an originally two-storey-or-more detached or semi-detached; max 18m overall height (17m terrace) and 7m added. Class AA runs through the same Prior Approval route with Neighbour Consultation and a 42-day window.

Full planning timelines: 8 weeks statutory target, 10-13 weeks realistic London average, 21-day consultation period, committee versus officer determination depending on objection count. Community Infrastructure Levy (CIL) applies to extensions creating more than 100 m² of new floorspace — most rear extensions stay under, but two-storey Class AA additions can push past it.

Party Wall Act 1996 — Section 3, 6, 10 reality

The Party Wall etc. Act 1996 is separate from planning. Planning permission does not excuse Party Wall obligations, and Party Wall notices do not grant planning. Both must be in place before work starts.

Section 3 fires when works affect the party wall directly — cutting a steel beam into the wall, raising it, or installing flashings that penetrate it. Notice period two months before commencement. Neighbour has 14 days to consent, dissent, or fail to respond (deemed dissent under Section 10).

Section 6 fires automatically on any excavation within 3 metres of a neighbour's building to a depth below their foundations, or within 6 metres if a 45-degree line drawn downwards from their foundation base would be crossed. Victorian terraces routinely fire Section 6 on both sides. Notice period one month. It does not matter that you are not touching the party wall — the excavation itself is the trigger.

Section 10 is the dispute engine. One Agreed Surveyor costs £900-£2,500 total for a typical rear extension. Two-surveyor (one per owner) routinely costs £2,000-£6,000 combined — the building owner pays both. A schedule of condition documents every crack on the neighbour's walls before work begins and becomes the reference for any damage claim.

Serve Section 3 two months before commencement and Section 6 one month before — run the timetables in parallel. Late service is the most common reason London rear extensions are stopped mid-build by county court injunction — we have seen a Wandsworth project halted seven weeks because the Section 6 notice was served the week the digger arrived.

Structural steels and the 5-7m open span trap

The design intent of most London rear extensions is to remove the existing kitchen rear wall and merge original kitchen, new extension, and (on wrap-arounds) the side return into one open-plan room. That requires a 5 to 7 metre clear structural opening where a load-bearing wall stood. It is a specialist engineering problem, not a decorative choice.

The standard solution is a steel rolled universal beam (UB) or rolled steel joist (RSJ) spanning from the party wall on one side to the opposite load-bearing wall or a new steel post. For a 5-metre span on a typical terrace the section is often a 203 × 203 UC used as a beam or a 254 × 146 UB — exact size depends on load, span, deflection, and fire resistance. These cannot be guessed. A Structural Engineer with Professional Indemnity Insurance of at least £1 million produces the signed calculation package, specifies padstones, details the connection into existing masonry, and issues calcs Building Control will review before the Completion Certificate.

Fabrication runs £1,200-£2,500 per beam delivered. Installation needs Acrow props at 1-metre centres holding the floor while the beam is set onto padstones. Building Control inspect the bearing, connections, fire protection (intumescent paint to 30 or 60 minutes), and deflection. On a wrap-around with intersecting beams and a moment connection, engineer fees rise to £3,500-£6,000 and fabrication to £4,000-£7,500.

Part L 2023 and the 2025 Future Homes Standard

Approved Document Part L covers conservation of fuel and power. Part L 2023 (in force from 15 June 2023) tightened U-values significantly over Part L 2013. Maximum U-values for new rear extensions:

• Walls — 0.18 W/m²K (was 0.28)
• Pitched roof — 0.16 W/m²K
• Flat roof — 0.18 W/m²K
• Windows and rooflights — 1.4 W/m²K (was 1.6)
• Doors — 1.4 W/m²K
• Floors — 0.18 W/m²K

Meeting the wall U-value on traditional cavity construction now needs 100mm PIR in the cavity plus 40mm PIR internally, or a 150-170mm full-fill mineral wool cavity. A specification drawn to 2013 standards will not pass a 2023 plan check.

Thermal bridging is separately assessed via Accredited Construction Details (ACDs) — standardised junction drawings at window reveals, lintels, floor junctions, and eaves. Deviating from ACDs forces a Psi-value calculation per junction through a SAP assessment.

The Future Homes Standard is the 2025 tightening (transitional provisions may push to 2026): no new gas boilers in new dwellings; heat-pump-ready flow temperatures at 55°C or below; walls to 0.15 W/m²K; windows to 1.2 W/m²K. For a 2026 rear extension, plan fabric to Part L 2023 but design heating heat-pump-ready — 55°C flow, larger radiators, UFH loop spacing for low-flow operation.

Building Control — LABC vs Approved Inspector

Every rear extension needs Building Control approval regardless of whether planning is required. Two routes:

Local Authority Building Control (LABC) — the borough's own inspection service. Fees typically £600-£1,100. Submissions run as Full Plans (plans approved before commencement, lowest risk) or Building Notice (inspections on site as work progresses, higher risk — any non-compliance found on site must be rebuilt).

Approved Inspectors — private-sector competitors offering statutory approval with typically faster plan review (5-10 working days versus 3-4 weeks). Fees £800-£1,600. Under the Building Safety Act 2022, all legitimate Approved Inspectors are listed on the Building Safety Regulator's register.

Mandatory inspections: foundation excavation before pour; damp proof course before slab; oversite concrete and drainage; superstructure including steel bearings; pre-plaster with insulation and fire-stopping visible; final Completion. The Completion Certificate proves the work was built to Regulations — without it, mortgage lenders flag at conveyancing and any future buyer's solicitor demands a retrospective Regularisation Certificate opening up finished work.

Cost bands: £2,800-£4,500/m² single-storey, side-return and wrap-around variants

Headline 2026 London rear-extension costs for structural shell plus weathertight envelope (before internal fit-out):

• Single-storey rear, basic spec — £2,800-£3,400/m². A 25m² extension is £70,000-£85,000 shell, or £75,000-£90,000 all-in including Party Wall Awards, engineer, and architect fees.
• Single-storey rear, mid spec — £3,400-£4,000/m². A 25m² extension is £85,000-£100,000 shell, £100,000-£120,000 all-in.
• Single-storey rear, high spec (premium glazing, bifold doors, rooflights, UFH) — £4,000-£4,500/m². A 25m² extension is £100,000-£112,500 shell, £125,000-£140,000 all-in.
• Side-return extension (typical 8-12 m²) — £45,000-£90,000 all-in. Per-m² rates are higher than rear extensions because the ratio of expensive perimeter detailing to cheap floor area is much worse.
• Wrap-around (side return plus rear, typically 25-40 m² combined) — £120,000-£250,000 all-in. Intersecting steels, moment connections, complex roof geometry, and two sets of Party Wall Awards.
• Two-storey rear (Class AA Prior Approval route) — £3,600-£4,500/m², typical 30m² at £145,000-£175,000 all-in.

Fit-out on top: kitchen furniture and installation £15,000-£45,000; flooring £4,000-£12,000; lighting, sockets, data £3,500-£8,000; MVHR if airtightness triggers Part F £3,500-£6,000; decoration £2,000-£5,000.

VAT on extension works is the standard 20 per cent rate. The 5 per cent reduced rate applies only in limited circumstances — renovating a dwelling empty for 2+ years, converting a house to flats, or certain energy-saving material installations. Standard extensions to an occupied house do not qualify for the 5 per cent rate. Zero-rated VAT applies only to genuinely new dwellings, which a rear extension is not. See HMRC VAT Notice 708 for the authoritative treatment.

Build programme: 16-24 weeks permission to completion

From the date planning or Prior Approval is issued, a typical single-storey rear extension runs 16-24 weeks to handover. The milestone pattern:

• Weeks 1-4 — Party Wall notices complete, tender awarded, contract signed, Structural Engineer calcs issued, steel order placed, site set-up.
• Weeks 4-8 — Demolition of existing rear wall, foundation excavation with Building Control inspection, concrete pour, damp proof course, oversite slab.
• Weeks 8-12 — Masonry shell to damp course, steel beam installation with Acrow propping, bearing sign-off, shell to roof-plate level.
• Weeks 12-15 — Roof structure, warm-deck build-up, rooflights, windows and doors — weather-tight shell achieved.
• Weeks 15-18 — First-fix M&E, insulation, vapour control layer, plasterboard, pre-plaster inspection.
• Weeks 18-22 — Plastering, second-fix, tiling, flooring preparation.
• Weeks 22-24 — Kitchen, decoration, final flooring, Completion Certificate.

A wrap-around extension runs 20-28 weeks. A two-storey rear extension runs 22-30 weeks. Winter programming (October through February) adds 2-3 weeks to the shell phase due to rain, short daylight, and concrete-curing delays. Schedule foundation and shell works March-September where possible.

What Baily verifies before any London match

Every London contractor Baily introduces has been verified across an eight-point checklist specific to the rear-extension regulatory stack. The contractor does not appear in a homeowner's match list without passing all eight:

1. Current NHBC or LABC Warranty registration — minimum 10-year structural warranty on completion.
2. Structural Engineer panel or named partner with Professional Indemnity Insurance £1 million minimum, current run-off, calculations signed and sealed.
3. Party Wall Surveyor panel registered with RICS or FPWS.
4. TrustMark registration — government-endorsed quality scheme with complaints-handling.
5. CHAS, SafeContractor, or Constructionline accreditation for CDM 2015 health and safety competence.
6. Public liability insurance £5 million minimum and employer's liability £10 million per the Employers' Liability (Compulsory Insurance) Act 1969.
7. Three recent completed projects with closed Party Wall Awards on file, photographic evidence, references — at least one in the homeowner's own borough.
8. Fair-payment terms — maximum 10 per cent deposit, stage payments against verifiable milestones, minimum 2.5 per cent retention held 12 months against defects.

Angi sends your details to 12 strangers. Baily sends them to one London contractor who has delivered rear extensions with closed Party Wall Awards in your postcode, whose Structural Engineer panel knows your borough's Building Control inspectors by name, and whose recent completed project in your ward is available to walk through before you sign.

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Frequently asked questions

Do I need planning permission for a 3m rear extension in London?

Not always — single-storey rear extensions up to 3m (semi/terrace) or 4m (detached) fall under GPDO Class A permitted development, but Conservation Areas, listed buildings, and Article 4 Directions remove those rights. Hackney, Camden, parts of Kensington & Chelsea and Islington have Article 4s — if your borough applies one, you need full planning even for a 2m extension. Check your Local Plan first and request a Lawful Development Certificate from the borough before design is finalised. The £120 fee is cheap insurance against a stop notice mid-build.

What happens if my neighbour objects to my Prior Approval application?

Objections must arrive within 21 days of the Planning Officer's notification. An objection does not automatically kill the application — it triggers the officer to consider amenity impact (loss of light, overshadowing, dominance, loss of outlook) on its merits. The officer can approve over objection or refuse. Refusal can be appealed to the Planning Inspectorate, but that adds 12-16 weeks. In Camden, Hackney, and Tower Hamlets, 35-50 per cent of Larger Home Extension applications attract at least one objection — plan for that rate when setting expectations with your family.

How much does a 5-metre open structural span actually cost?

For a 5-metre clear span carrying typical floor-over-and-roof loads on a London terrace: Structural Engineer's calculation package £1,200-£2,500, steel fabrication £1,200-£2,500 per beam delivered, installation including temporary propping and Building Control inspection £2,000-£3,500. Total steelwork £4,500-£8,500 on a single-opening rear extension. On a wrap-around with intersecting beams and a moment connection, £10,000-£18,000. Never accept a quote that lumps "steels" into a single line without itemising engineer, fabricator, installation, and padstones.

Can I avoid the Party Wall Act if my neighbour is friendly?

No. The Act is statutory. Serving the notice is mandatory regardless of the relationship. Your neighbour can issue written consent within 14 days, which removes the need for a surveyor and a formal Award, and that is the cheapest route. But the notice itself is non-negotiable — omit it and a county court can grant an injunction stopping works the moment the neighbour applies, regardless of how friendly they seemed before the excavator arrived. Serve Section 6 one month before, Section 3 two months before, and keep the written consent or Award on file.

What is the real difference between Building Notice and Full Plans for Building Control?

A Building Notice allows work to start without Plans being formally approved — the inspector signs off each stage on site. The risk is that any non-compliance discovered on site has to be rebuilt; if an inspector decides a foundation is inadequate after the floor slab has been cast, you are opening up finished work. A Full Plans application submits drawings and calculations for approval before commencement. Approval takes 3-5 weeks but eliminates the rebuild risk. For any rear extension involving steel beams or excavation near neighbours, Full Plans is the only safe route. Accept the three-week delay; it pays for itself.

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Citations and references