The short answer
In 2026, a UK homeowner can build a single-storey rear extension up to 4 m (terraced) or 6 m (semi-detached) or 8 m (detached) without applying for full planning permission, subject to height, eaves, coverage, and materials conditions under Class A of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 (the GPDO). For a terraced house the limit is straightforward; for a semi-detached or detached house the larger-home prior approval process applies, which is a 42-day LPA check on neighbour amenity and highways only, not a full planning application.
For the full PD rules including the height, eaves, coverage, and materials conditions, see Do You Need Planning Permission for an Extension? UK 2026 Rules and Pillar 1: The Complete UK Guide to Planning Permission for a Loft Conversion (2026) for the loft equivalent.
What you can build under PD in 2026
A 2026 PD extension is, in plain English:
- A single-storey ground-floor extension to the rear or side of a house.
- Up to 4 m deep on a terraced house (no prior approval needed).
- Up to 6 m deep on a semi-detached house (larger-home prior approval).
- Up to 8 m deep on a detached house (larger-home prior approval).
- Up to 4 m in total height, with eaves no higher than the existing house eaves.
- With eaves no more than 3 m where the extension is within 2 m of a boundary.
- Built in materials similar in appearance to those used on the original house.
- Without verandas, balconies, raised platforms, or a chimney on a side elevation.
- Without a double-storey element (a single-storey extension with a pitched roof is fine; a true two-storey extension is not).
A typical 2026 PD extension on a 3-bed semi-detached house in the Midlands is a 5 m × 4 m rear kitchen-diner (20 m²), with a flat or low-pitched roof set 200 mm below the 3 m eaves cap (to allow for the boundary rule), built in matching brick. That is the most common PD build in the UK.
What about side extensions?
Class A also permits side extensions, but the rules are tighter:
- A side extension on its own (no rear component) must not extend beyond the side wall of the original house by more than half the width of the original house.
- The extension must not be forward of the principal elevation (i.e. it cannot project past the front of the house).
- A two-storey side extension needs a full planning application, even if the depth is small.
A typical 2026 PD side return extension on a Victorian terrace is the side return between the back of the house and the boundary wall — usually 2–3 m wide and 4–5 m deep, often combined with a small rear extension to form an L-shape. The Party Wall Act 1996 usually applies because the work touches the boundary.
What about the larger-home prior approval?
For a semi-detached or detached house extending more than 4 m at the rear, the larger-home prior approval process under paragraph A.4 of Class A is required. The process is:
- Submit a prior approval application to the LPA — the form is the same as a householder planning application, with a fee that is currently free.
- The LPA consults neighbours for 21 days, then considers the proposal on neighbour amenity (overlooking, overshadowing, loss of light) and transport and highways impact only.
- The LPA issues a decision within 42 days of the application being validated. Possible outcomes are "no prior approval required", "prior approval required (with conditions)", or a refusal on the limited grounds above.
- If "no prior approval required" or "prior approval granted" is issued, you can start work under PD as if the original Class A applied. If the LPA refuses, you would need a full householder planning application to build the design.
In practice, the LPA rarely refuses a prior approval application on neighbour amenity grounds for a typical 4–6 m rear extension on a semi-detached house. Most refusals happen where the extension is unusually deep (closer to 6 m than to 4 m) and a neighbour has raised a substantive loss-of-light objection.
What is not allowed under PD
Five items are commonly outside the Class A PD envelope and require a full householder planning application:
- Double-storey extensions within 2 m of a boundary. Class A.1(k) excludes any enlargement within 2 m of a boundary that would extend more than 3 m beyond the rear wall of the original house.
- Front extensions. Any extension forward of the principal elevation of the house.
- Verandas, balconies, or raised platforms. All three are excluded from Class A. A Juliet balcony on a first-floor extension is also a Class A breach.
- Materials that are not similar in appearance to the original house. A grey-rendered extension on a red-brick house is a Class A breach.
- Extensions that exceed the 50% curtilage coverage rule. The total area of all outbuildings, previous extensions, and the new extension within the original curtilage must not exceed 50% of the total curtilage area (excluding the original house footprint).
If any of these conditions are breached, the only way to build the design is a full householder planning application. The LPA can then apply normal design, appearance, and neighbour-amenity tests.
What about conservation areas and listed buildings?
A growing number of conservation-area LPAs have an Article 4 Direction that removes PD for rear and side extensions. The Direction is a local planning policy that the LPA can put in place without modifying the GPDO; it just withdraws the PD right from the affected properties. The Direction is registered as a local land charge, and your solicitor's pre-purchase searches will pick it up.
If the property is in a conservation area with an Article 4 Direction for extensions, every extension needs a full householder planning application regardless of size, depth, or height.
A listed building needs Listed Building Consent in addition to any planning application. Listed Building Consent is a separate regime and is applied for through Historic England (for Grade I and Grade II*) or the LPA (for Grade II).
When you do not need any approval at all
A 2026 extension is fully exempt from any planning or Building Regulations approval if all of the following are true:
- It is a small outbuilding (e.g. a garden shed) under 15 m³ (single-storey) or under 10 m³ (with a pitched roof), and it sits more than 2 m from the boundary (or 1 m if under 2.5 m tall).
- It is not used as a sleeping room (e.g. a garden office used as a bedroom is not exempt).
- It is not in front of the principal elevation of the house.
- It does not include a flue, chimney, or fixed water supply.
A 2.5 m × 3 m garden shed on a 5 m × 4 m base in the back garden, used as a workshop, is the most common fully-exempt 2026 build. A garden office with electricity, a fixed water supply, or sleeping accommodation is a different regime and may need either a full planning application or a Building Regulations application.
Building Regulations still apply
A 2026 PD extension is still subject to Building Regulations under the relevant Approved Documents (Part A structure, Part B fire safety, Part F ventilation, Part K stairs and guards, Part L conservation of fuel and power, Part N condensation). The homeowner needs a Full Plans or Building Notice application to the LPA Building Control team (or an Approved Inspector), with the building control fees and inspections that entails.
A typical 2026 Building Notice fee for a small single-storey extension is in the region of £250–£500, with Full Plans fees of £400–£900 for a more complex project. Approved Inspector fees are similar.
Where a topographical survey fits in
A PD extension still needs a topographical survey of the existing site, just as a full-planning extension does. The architect needs to know the levels of the garden, the boundary positions, the drainage runs, and any trees that might affect the foundation design. A topographical survey for a typical house extension in 2026 costs in the region of £800–£1,200 + VAT for a small residential site.
A PD extension that turns out to be 100 mm over the depth limit, or 50 mm above the height cap, can be caught at design stage if the architect has an accurate topographical survey and a measured building survey to work from. A £1,500 survey is the difference between a PD build and a forced full planning application at £548 plus 8 weeks of delay.
icelabz provides RICS-compliant topographical and measured building surveys across London and the South East, with deliverables in 2D CAD and (optionally) Revit BIM and point cloud. Contact us for a fixed-fee quote and a typical 10–15 working-day turnaround.