Free Tool
How Big Can My Extension Be?
Find out the maximum permitted development dimensions for your extension. Enter your address and we'll automatically detect your property type and designation status — then get an instant result based on Class A of the GPDO 2015 (England).
Class A depth limits at a glance
4m
Detached house, single storey (standard PD)
3m
Semi-detached / terraced, single storey (standard PD)
8m
Detached house, single storey (prior approval scheme)
6m
Semi-detached / terraced (prior approval scheme)
3m
Any house type, two-storey (no prior approval available)
½ width
Max width of a single-storey side extension
Depth is always measured from the original rear wall of the house as first built — not from any existing extension. The prior approval scheme is not available on conservation areas, AONBs, or National Parks.
Your address
Property type
Context
Extension type
Step 1 — What's the address?
Enter your property address and we'll automatically detect the house type and whether it's on designated land — saving you having to answer those questions manually.
Can't find your address?