Maximum Extension Size Without Planning Permission in London (2026)
Exact permitted development size limits for every extension type - rear extensions, lofts, side extensions, and the Prior Approval scheme that allows larger rear extensions.
TL;DR - The Quick Answer
Permitted development size limits in London: single-storey rear extensions up to 3m deep (terraced/semi-detached) or 4m deep (detached), maximum 4m high. The Prior Approval scheme (£120 fee) doubles this - 6m (terraced/semi) or 8m (detached) - using a neighbour notification process instead of a planning application. Loft conversions: up to 40m³ volume increase (terraced) or 50m³ (detached/semi). Most PD rights are restricted in conservation areas (no prior approval, no side extensions, no dormers under PD), but standard rear extensions (3m/4m) are still permitted. Article 4 direction zones may remove PD rights entirely.
Permitted Development Size Limits at a Glance
| Extension Type | Terraced / Semi | Detached | Prior Approval max |
|---|---|---|---|
| Single-storey rear | 3m deep, 4m high | 4m deep, 4m high | 6m / 8m (with notification) |
| Two-storey rear | 3m deep | 3m deep | Not available |
| Side extension | Half house width, 4m high | Half house width, 4m high | Not available |
| Loft conversion | 40m³ volume | 50m³ volume | Not available |
| Outbuilding | 50% of garden, 4m high (dual pitch) or 3m (other) | 50% of garden, 4m high (dual pitch) or 3m (other) | Not available |
| Porch | 3m² footprint, 3m high | 3m² footprint, 3m high | Not available |
Source: MHCLG Permitted Development Rights for Householders (Technical Guidance) • Data as of 2026
These are the national limits - valid across England. In London, conservation areas and Article 4 directions remove or restrict these rights for a large proportion of properties. Always verify your specific address before assuming limits apply.
The Prior Approval Scheme: How to Build a Larger Extension
The Prior Approval scheme allows larger single-storey rear extensions than standard PD - without a full planning application:
- Terraced and semi-detached houses: up to 6m deep (vs 3m standard PD)
- Detached houses: up to 8m deep (vs 4m standard PD)
- Single-storey only, maximum 4m high
- Application to council: £120 fee
- Neighbour notification period: 42 days
- Objections from neighbours can result in a full planning assessment
- Not available in conservation areas or Article 4 direction zones
In practice, the Prior Approval scheme is underused - most homeowners don't know it exists. A 6m deep kitchen extension on a terraced house can proceed without planning permission via this route. Mayfair Studio handles Prior Approval applications as part of its service.
What Counts Toward the Size Limits?
The permitted development limits apply to the total extension from the original rear wall - not just the new addition. Key points:
- Depth is measured from the original rear wall (as built, not as subsequently extended)
- If a previous owner extended, that extension counts toward your limit
- A conservatory or previous extension already at 3m means you have no remaining PD depth
- The 40m³/50m³ loft limit is cumulative - all roof conversions count together
- Volume calculations are based on external dimensions
Height and Other Key Limits
| Restriction | Limit |
|---|---|
| Maximum height of single-storey extension | 4m (pitched roof) or 3m (flat roof, if within 2m of boundary) |
| Maximum height of side extension | 4m (no higher than original house) |
| Maximum height of extension at boundary | 3m (eaves and ridge) |
| Loft: cannot extend beyond front roof slope | - (any protrusion on front slope requires planning) |
| Loft: maximum height | Same as existing ridge height |
| Maximum percentage of garden covered | 50% of curtilage (all outbuildings combined) |
Checking Your Specific Property
These rules set the maximum possible under PD - but your specific property may have additional restrictions:
- Conservation area? PD rights removed for most extensions - planning permission required
- Article 4 direction? Specific PD rights removed - check your specific postcode
- Listed building? Listed building consent required - no PD rights apply at all
- Previous extensions? Count toward your remaining PD depth - check with an architect
- Neighbour covenant? May restrict extensions beyond planning rules - check title deeds
The maximum single-storey rear extension under permitted development in London is 3m deep for terraced and semi-detached houses, 4m deep for detached. The Prior Approval scheme (£120 fee) extends this to 6m or 8m via a 42-day neighbour notification process. Loft conversions are capped at 40m³ (terraced) or 50m³ (detached/semi). Prior approval is not available in conservation areas, though standard PD rear extensions (3m/4m) remain permitted. Article 4 direction zones may remove PD rights entirely - these affect over 40% of London residential properties.
Frequently Asked Questions
What is the maximum extension size without planning permission in London?
The maximum single-storey rear extension under permitted development in London is 3m deep for terraced and semi-detached houses, 4m deep for detached houses (max 4m high). The Prior Approval scheme extends this to 6m (terraced/semi) or 8m (detached) - £120 application fee, using neighbour notification instead of a planning application. Loft conversions are capped at 40m³ volume (terraced) or 50m³ (detached/semi). Prior approval is not available in conservation areas, and Article 4 zones may remove PD rights entirely.
How big can my extension be without planning permission in London?
Without planning permission (under permitted development): single-storey rear up to 3m deep (terraced/semi) or 4m deep (detached). Via Prior Approval scheme (£120 fee): up to 6m deep (terraced/semi) or 8m deep (detached). Side extensions: up to half the width of the original house. Lofts: up to 40m³ (terraced) or 50m³ (detached/semi). Prior approval is not available in conservation areas. Article 4 direction zones may remove PD rights entirely.
What is the 3m rule for extensions in London?
The 3m rule refers to the maximum permitted development depth for single-storey rear extensions on terraced and semi-detached houses in London. Extensions up to 3m deep from the original rear wall don't need planning permission. For detached houses, the limit is 4m. The Prior Approval scheme (£120 fee, 42-day determination) extends these to 6m and 8m respectively via a neighbour notification process. Prior approval is not available in conservation areas.
Can I build a 6m extension without planning permission in London?
Yes - under the Prior Approval scheme (also called the Larger Home Extension scheme), terraced and semi-detached houses can build rear extensions up to 6m deep without full planning permission. Detached houses can build up to 8m deep. The scheme uses a neighbour notification process: you notify the council and neighbours, who have 42 days to object. If no valid objections are raised, the extension can proceed. Not available in conservation areas or Article 4 zones.
Do I need planning permission for a 4m rear extension in London?
For a detached house: no - a 4m rear extension qualifies as permitted development (no planning needed) outside conservation areas and Article 4 zones. For a terraced or semi-detached house: standard PD limits are 3m deep, so a 4m extension would require either Prior Approval (£120 fee, allows up to 6m) or full planning permission. Check your specific address for conservation area or Article 4 restrictions that may apply.