“Close to the boundary” and “entitled to build across it” are different questions. [1] [2]
Check height in the correct way
A single-storey rear extension under Class A must not exceed 4m overall height. The maximum eaves height is 3m where any part of the proposed extension is within 2m of the boundary. The applicable limits related to the existing house's roof and eaves must also be satisfied. [1]
Eaves height is not interchangeable with the height of a parapet or the highest part of a roof. Ask the designer to show ground level, eaves and the highest roof point on a section, using the definitions in the technical guidance. On sloping ground, the guidance specifies how ground level is established; it is not simply the floor level chosen for the extension. [1]
These are permitted development tests. If the project needs a planning application, the council's design and neighbour-amenity assessment is a separate exercise. Do not treat 3m as an automatic entitlement to construct a boundary wall in every situation. [3]
Locate the legal boundary
Resolve an uncertain boundary before placing a wall on it. A party wall surveyor's award cannot determine a dispute about where the boundary lies. Planning permission also does not itself establish ownership rights. [2]
Show the full construction on the drawings—not only the inner face of the finished room. Consider wall thickness, roof projections, rainwater details and the foundation arrangement. Any use of neighbouring land needs a lawful basis; it should not appear for the first time when excavation starts. [2]
New walls and existing party walls differ
A new wall astride the boundary requires the adjoining owner's written consent. A wall wholly on your land but built up to the boundary can still require a line-of-junction notice. Work to an existing party wall, such as raising it or cutting into it for a beam, is governed by different provisions. [2]
Excavation notices can also apply even where the new wall is set back. The statutory tests depend on distance and foundation levels, not simply on whether a wall touches the boundary. [2]
Allow for engineering and fire safety
A boundary can constrain the location of a conventional foundation. An offset foundation has a different loading arrangement and needs proper design; it is not a matter of moving an otherwise standard trench sideways. [4]
Building regulations also address fire spread between buildings, including external walls, openings and roof construction near boundaries. Planning acceptance of a location does not establish the correct technical construction. [5]
Agree the physical line, lawful rights, engineering detail and approvals together before work starts. Where ownership or access is disputed, obtain project-specific legal advice rather than asking the builder to choose a position.