Rear Extension Depth Calculator
Before you hire an architect or submit plans, check whether your rear extension falls within the permitted development depth limits. This calculator covers standard PD, the Larger Home Extension (Prior Approval) scheme, and double-storey rules under Class A of the GPDO 2015.
Class A depth limits at a glance
Depth is always measured from the original rear wall of the house — not from any existing extension. The “original house” means the house as first built, or as it stood on 1 July 1948.
Calculate your maximum depth
Answer three or four questions to get an instant result. You do not need drawings — just know your house type and proposed extension depth.
Measure from the original rear wall of your house, in metres. If you already have an extension, measure from where the original wall was, not from the existing extension.
In designated areas the standard PD limit still applies but additional restrictions affect side and double-storey extensions. Check your council’s website or use our full PD checker.
Other rules depth alone does not cover
- Height: Single-storey extensions must not exceed 4m overall. If within 2m of a boundary, eaves must not exceed 3m.
- Materials: External materials must be similar in appearance to the existing house.
- Coverage: All extensions and outbuildings combined must not cover more than 50% of the garden area.
- Party wall: If you are building near a shared wall or boundary, you may need to serve notices under the Party Wall etc. Act 1996.
- Building regulations: Almost always required, covering foundations, structural safety, insulation, drainage, and electrics.
- This is a depth check only — use our full PD checker with your address to confirm all Class A rules are met.