The short answer
Most single-storey rear extensions on a UK house in 2026 are permitted development (PD) and do not need a planning application. Double-storey extensions and any extension on a terraced house usually need a full householder planning application. The 2026 PD rules are governed by Class A of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 (the GPDO), and the depth limits depend on the type of house.
| House type | 2026 PD single-storey rear extension max depth | |------------|-----------------------------------------------:| | Detached | 8 m (larger-home prior approval required) | | Semi-detached | 6 m (larger-home prior approval required) | | Terraced | 4 m (no prior approval) |
For a full guide to the loft equivalent of these rules, see Pillar 1: The Complete UK Guide to Planning Permission for a Loft Conversion (2026). For the cost side of the project, see Pillar 3: UK House Extension Cost Guide 2026.
The Class A depth rules in plain English
The headline rule under GPDO 2015 Class A is that the depth of the extension (measured from the original rear wall of the house to the rear wall of the extension) must not exceed:
- 4 m for a terraced house.
- 6 m for a semi-detached house.
- 8 m for a detached house.
For terraced houses, the limit is straightforward: a 4 m rear extension is PD and no prior approval is needed. For semi-detached and detached houses, the rule is the same on paper, but the larger-home prior approval process under paragraph A.4 means you must apply to the LPA for a "no prior approval required" determination before starting work.
The LPA has 42 days to consider the prior approval, and it can refuse only on the grounds of neighbour amenity (overlooking, overshadowing, loss of light) or transport and highways impact. It cannot refuse on design or appearance grounds — that is the trade-off for the larger depth allowance.
Height and eaves rules
The Class A height rules are layered on top of the depth rules:
- No part of the extension may be higher than the highest part of the existing roof. A.1(c).
- A single-storey rear extension must not exceed 4 m in height. A.1(f)(ii) and A.1(g)(ii).
- The eaves of the extension must not exceed the height of the eaves of the existing house. A.1(d).
- Where any part of the extension is within 2 m of a boundary, the eaves of that part must not exceed 3 m. A.1(i).
The 3 m eaves rule within 2 m of a boundary is the most common Class A breach on terraced houses, because the extension often runs right up to the boundary wall and the standard 2.4–2.7 m house eaves exceed the 3 m cap once the new extension eaves are added. The practical answer is to design the extension with a flat or low-pitched roof, with the eaves set 200 mm below the 3 m cap to allow for tolerance.
The 50% garden coverage rule
A common misconception is that a rear extension can use up to 50% of the garden. The actual rule under Class A is more subtle: the total area of all outbuildings, previous extensions, and the new extension within the original curtilage of the house must not exceed 50% of the total curtilage area (excluding the original house footprint).
For a typical 3-bed semi with a 50 m² house footprint and a 100 m² rear garden, the 50% rule means the new extension plus any existing outbuildings (shed, garage, summerhouse) cannot exceed 50 m². A 4 m × 5 m (20 m²) extension would be fine, but a 6 m × 8 m (48 m²) extension would push the total over the 50% cap if the existing shed and garage are still standing.
Materials, windows, and other conditions
Class A includes several other conditions that frequently force a full planning application:
- Materials. The extension must be built in materials similar in appearance to those used on the original house. A grey-rendered extension on a red-brick house is a Class A breach.
- No windows on side elevations facing a neighbour. Side-facing windows above ground level are a Class A breach. Obscure-glazed side windows are fine.
- No verandas, balconies, or raised platforms. All three are excluded from Class A. A Juliet balcony on a first-floor extension is also a breach.
- No chimney, flue, or soil vent pipe on a side elevation facing a neighbour. Restricted by Class A.1(j).
If any of these Class A 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.
When an extension always needs full planning
Five situations always force a full householder planning application, regardless of the depth and height of the extension:
- The house is a flat or maisonette. Flats have no Part 1 PD rights.
- The property is in a conservation area, AONB, National Park, the Broads, or a World Heritage Site, and your local authority has an Article 4 Direction that removes PD for extensions.
- The building is listed. Listed Building Consent is required in addition to any planning application.
- The design exceeds the Class A depth, height, eaves, coverage, or materials conditions.
- The extension is double-storey (or above) and sits within 2 m of a boundary. Class A.1(k) excludes double-storey extensions within 2 m of a boundary from PD — they always need a full application.
How to apply for a householder planning application
The process is the same as for a loft conversion — see Pillar 1 for the step-by-step guide. The headline numbers for 2026:
- Application fee: £548 from 1 April 2026 (CPI-indexed from £528 in 2025).
- Determination period: 8 weeks from validation, with a 16-week planning guarantee longstop.
- Larger-home prior approval (for 6 m / 8 m extensions): 42 days, neighbour amenity and highways only.
- Appeals: free for householder applications, lodged with the Planning Inspectorate.
What if you build without permission?
If you carry out an extension that needed planning permission but you did not get it, the LPA can serve an enforcement notice requiring you to undo the work or apply retrospectively. Retrospective applications are routinely refused on the same grounds the original application would have been.
The enforcement immunity period in England is 10 years for all breaches of planning control from 25 April 2024 onwards, following amendments to section 171B of the Town and Country Planning Act 1990 by the Levelling-up and Regeneration Act 2023. Extensions substantially completed before 25 April 2024 may still benefit from the old 4-year rule for operational development.
The full enforcement consequences are covered in Pillar 1.
Where a topographical survey fits in
A house extension cannot be designed or quoted accurately without a topographical survey of the existing site. 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.
icelabz provides RICS-compliant topographical surveys across London and the South East, with deliverables in 2D CAD and (optionally) Revit BIM. Contact us for a fixed-fee quote and a typical 10–15 working-day turnaround.