Permitted development is a national grant of planning permission, not an absence of planning rules. [1]
Establish whether the house has the relevant rights
The householder extension rules are for houses, not flats or maisonettes. Check previous permissions and any Article 4 direction because these can remove rights. Houses created through certain change-of-use permitted development routes also need particular care. Conservation areas and other protected locations have additional restrictions. [1] [2]
Identify the original walls before measuring. The original house means its form on 1 July 1948, or as first built if later. Include previous extensions when the rules require the total enlargement to be assessed. A conservatory left by a previous owner is not automatically outside the calculation. [1]
Choose between the three main routes
For an ordinary single-storey rear extension, the headline permitted development depth limit is 3m for an attached house or 4m for a detached house. There are separate limits on overall height, eaves, boundary proximity and the proportion of the site covered by buildings. [2]
A larger single-storey rear extension can potentially project up to 6m for an attached house or 8m for a detached house through the prior-approval procedure. This route is not available on article 2(3) land, including conservation areas, or Sites of Special Scientific Interest. It is not a way to bypass the remaining Class A limitations. [1] [3]
Where neither route fits, prepare a householder application for the extension. The council then considers the proposal against the relevant planning framework, rather than treating permitted development dimensions as the largest extension that could ever receive permission. Borough design guidance is important to that assessment. [4]
What evidence should be prepared?
Ask for a measured plan showing original and later walls, the proposed footprint, heights and relevant boundaries. Include neighbouring windows and buildings where these affect the design assessment. For a permitted development proposal, consider obtaining a Lawful Development Certificate to document the planning position; the certificate route tests lawfulness, not whether the council likes the design. [1]
Check the householder Biodiversity Net Gain exemption rather than assuming every planning application needs biodiversity units. GOV.UK's exemption guidance, updated on 6 August 2026, expressly includes householder extensions. [5]
Complete the other approvals before starting
Building regulations, party wall procedures and any sewer agreement remain separate. The planning route does not establish that a foundation or structural opening is safe, or authorise encroachment onto neighbouring land. [6] [7] [8]
From 31 October 2026, the national delegation scheme will normally require householder applications to be decided by officers, subject to statutory exceptions. It changes who decides relevant applications, not whether your extension satisfies the planning requirements. [9]