Permitted Development Rules for Side Extensions: What You Can Build

**Class A, Part 1, Schedule 2, GPDO 2015 — single-storey side extensions**:

Single-storey side extensions are covered by Class A of Part 1 of Schedule 2 to the GPDO 2015. To be PD, a single-storey side extension must satisfy ALL of the following conditions:

• **Condition A.1(e) — Maximum height**:
• •Maximum height of the extension: 4.0m
• •Maximum height of the eaves: 3.0m

• **Condition A.1(f) — Width limit (the most commonly triggering condition for side extensions)**:
• •The side extension must not exceed half the width of the original dwellinghouse
• •'Original' means the building as it was on 1 July 1948, or as built if later — not the current width after any subsequent additions

*Example*: An original house is 5.0m wide at the front. The maximum PD side extension width is 2.5m (half of 5.0m). A 3.0m wide side extension on this house would require planning permission.

• **Condition A.1(h) — No forward of the principal elevation**:
• •No part of the side extension may project forward of the principal elevation (the front wall of the house) or the side elevation of the original house facing a highway

• **Condition A.1(j) — On a designated site (Conservation Area, National Park, AONB, etc.)**:
• •A side extension is NOT PD on a dwelling in a Conservation Area, World Heritage Site, National Park, or Area of Outstanding Natural Beauty
• •In a Conservation Area, any side extension (whether single-storey or two-storey) requires full planning permission

• **Condition A.1(k) — Not to consist of or include the installation of a veranda**:
• •As with rear extensions, the side extension cannot include a balcony, veranda, or raised platform

**The half-width rule in detail**:

The half-width condition is the most commonly misunderstood and most commonly triggering condition for side extensions. Key clarifications:

*What counts as the 'original' width*: The original width is the width of the main building mass on the date of the original construction (or 1 July 1948 if earlier). An existing side extension added after 1948 does NOT increase the original width. If a 1.0m side extension was previously added to a 5.0m original house, the original width is still 5.0m — but the remaining PD allowance for further side extension is reduced (any PD extension must be no wider than the original half-width, minus any area already built under PD).

*How width is measured*: Width is measured from the outer face of the original side wall to the outer face of the new extension wall.

**Double-storey side extensions**:

Two-storey side extensions are NOT Permitted Development — they always require full planning permission. Class A permits only single-storey extensions (Condition A.1(e) maximum height 4.0m refers to the eaves and ridge, but the intent is single-storey). A two-storey side extension has an eaves height well above 4.0m and a total height (to ridge or flat roof) above 4.0m — both conditions exclude it from PD.

**Side return extensions (filling the side alleyway in terraced houses)**:

In London Victorian and Edwardian terraces, the 'side return' is the L-shaped or stepped area to the side of the rear reception room — the area covered by the traditional outrigger or scullery extension at the rear, leaving a narrow alleyway to the side of the rear outrigger. 'Side return extension' in London typically means filling this side alleyway with a new extension, running from the existing outrigger to the party wall boundary.

• A side return extension in a terraced house is usually measured as a combination of rear and side components:
• •The rear component (extending the rear outrigger further back) is assessed against the rear extension PD rules (Class A or Class B depending on depth)
• •The side component (filling the alleyway) is assessed against the side extension PD rules

In practice, many side return extensions in London terraced houses are PD for the side component (typically 0.5–1.0m wide, well within the half-width limit) but may need Planning Permission for the rear component if the total depth exceeds the rear extension PD limit (3.0–6.0m).

**Conservation Areas and side extensions — no PD**:

As noted in Condition A.1(j), side extensions are NOT PD in Conservation Areas. In London, a significant proportion of the residential housing stock is in Conservation Areas — particularly Victorian terraces in inner London boroughs. If a property is in a Conservation Area, any side extension requires full planning permission, regardless of its size or whether it would otherwise satisfy the PD conditions.

**Article 4 Directions — additional restrictions beyond Conservation Areas**:

Many London boroughs have Article 4 Directions that remove PD rights for side extensions even outside Conservation Areas. Notable examples:

*Islington*: The whole borough is largely covered by Conservation Area or Article 4 Direction restrictions; side extensions in most residential streets require planning permission.

*Hackney*: Multiple Article 4 Directions covering side extensions across the borough; many residential areas have no PD right for side extensions.

*Camden*: Similar extensive Article 4 coverage; most residential areas require planning permission for side extensions.

*Lambeth, Southwark, Lewisham*: Widespread Conservation Area coverage; PD side extensions are limited to areas not in Conservation Areas and not subject to Article 4 Directions.

*Outer London (Hounslow, Hillingdon, Barnet, Havering)*: Generally fewer Article 4 Directions on side extensions; PD rights more widely available but still subject to the half-width and other conditions.

**How to confirm whether PD applies to a specific property**:

*Step 1 — Check if the property is in a Conservation Area*: The LPA's planning portal mapping tool shows Conservation Area boundaries. Alternatively, search '[borough name] conservation area map' on the LPA website.

*Step 2 — Check for Article 4 Directions*: The LPA's planning portal or planning policy pages list all operative Article 4 Directions. Many boroughs now have mapping tools that overlay Article 4 Directions on the base map.

*Step 3 — Check the planning history of the property*: Has any PD right been used already (previous side extension)? Previous PD additions reduce the remaining PD allowance.

*Step 4 — Consider a Certificate of Lawful Development (Proposed)*: If there is any doubt, a CLD application (£206 fee, 8 weeks) provides formal written confirmation that the proposed extension is PD. This is essential protection if the property will be sold within a few years of the works.

**When planning permission is required for a side extension**:

Where a side extension requires planning permission (Conservation Area, Article 4 Direction, two-storey, exceeds half-width limit), the LPA assesses the proposal against:

*The 'terracing effect'*: Many LPAs are concerned about the 'terracing effect' — where a side extension on a semi-detached house creates the visual appearance of a terraced property by eliminating the visual gap between houses. Policies often require a side extension on a semi-detached house to be set back from the front building line and to be clearly subordinate to the original house in width and height — so that the gap between houses is still readable from the street.

*Highway visibility and access*: Side extensions that encroach on access to the rear garden (blocking the side gate) or that affect highway visibility splays (sight lines at the driveway crossing) may be refused. The extension must not obstruct the required visibility splay distances for vehicles leaving the property.

*Scale relative to the original dwelling and the street*: The extension must be in proportion to the original house and appropriate in the context of the street. A side extension that is wider than the original house or higher than the main roof is typically refused.

*Materials*: In Conservation Areas (where planning is required), materials must complement the existing building and the area's character (see Conservation Area extension design guide).

**Planning application drawings for a side extension**:

• •Location plan (1:1250)
• •Block plan (1:500) — essential for showing the extension's relationship to the site boundary and highway
• •Existing and proposed ground floor plan
• •Existing and proposed elevations (front, side, and rear elevations)
• •Existing and proposed sections

For a straightforward single-storey side extension requiring planning permission, the architectural and planning drawings can typically be prepared in 3–5 days; the planning application fee is £258 (England, 2025).