Quick answer: A rear or side extension can require notice because it builds at the boundary, cuts into or exposes a party wall, or excavates close to and deeper than a neighbour’s foundations. Planning approval does not remove Party Wall etc. Act duties.
This article concerns England and Wales. It provides general information, not legal advice. The application of the Party Wall etc. Act 1996 depends on the facts, ownership and proposed work.
Building at the boundary
Section 1 can apply when constructing a new wall at or astride the line of junction. Building astride the boundary requires the adjoining owner’s express consent; if that consent is refused, the building owner may usually build wholly on their own land, subject to the Act and other property rights.
Connecting to an existing wall
Cutting flashings, beams or other structural elements into a party wall, raising it or exposing it to weather can fall under section 2. The design should explain protection, making good and access requirements.
Extension foundations
Section 6 may apply where excavation is within three metres and lower than the neighbour’s foundations, or within six metres where the defined 45-degree test is met. Actual foundation levels matter; distance alone is not the complete test.
A sensible extension checklist
Confirm the boundary, obtain structural and foundation details, identify all adjoining owners, serve every necessary notice and leave enough time for consent or an award before the contractor starts.
Frequently asked questions
Does permitted development avoid the Act?
No. Permitted development concerns planning; Party Wall duties remain separate.
Can foundations project onto a neighbour’s land?
Special foundations generally require written consent. Other foundations and access questions need project-specific advice.
Authoritative sources and further help
Read the official GOV.UK explanatory booklet and the Party Wall etc. Act 1996 on legislation.gov.uk. For the wider process see our building owners guide, adjoining owners guide and notice guidance.
Reviewed: 16 July 2026. Seek advice from a suitably experienced party wall surveyor or solicitor where the facts or legal position are disputed.