Answer in brief
You can build right up to the boundary, but each step closer brings a rule. Within 2 m the eaves must stay under 3 m for permitted development, within 1 m the wall must resist fire, and building on the line or digging near a neighbour's foundations means serving party wall notice.
No law sets a minimum gap between an extension and the boundary. You can build with the outside face of your wall on the line itself. What changes as you get closer are the conditions, and they come from four different places: planning, the Party Wall etc. Act 1996, the fire safety part of the building regulations, and plain property law.
Planning: the 2 m line
English permitted development caps the eaves at 3 m on any stretch of an extension that comes closer than 2 m to a boundary. For a one-storey back extension, that is the only rule tied to distance. Outside 2 m the eaves can rise to match the house, up to the 4 m overall height limit.
For a full-width extension on a semi or terrace, that means keeping the walls along both sides low. A flat-roofed kit with a 3 m parapet or eaves meets it comfortably. If you want a pitched roof sloping up to the house, check the eaves height on the side walls, not just at the front.
If you need planning permission anyway, the council looks at light and outlook for the neighbour rather than a fixed distance. A tall blank wall right on the line of a neighbour’s patio is the classic reason for refusal.
Party wall notice
In England and Wales, the 1996 Party Wall Act comes into play if you:
- build a new wall on or astride the boundary line,
- excavate below the level of a neighbour’s foundations anywhere within 3 m of their building, or
- cut into or alter an existing party wall, for example to tuck a beam into it.
You must serve written notice before you start: at least one month for a new wall on the line or for excavation, and two months for work to an existing party wall. The neighbour can agree, or dissent, in which case surveyors are appointed to settle an award. Most small rear extensions on semis and terraces trigger the excavation rule, because new footings are usually deeper than old ones.
Fire: the 1 m line
Approved Document B controls how fire can spread between buildings. A wall less than 1 m from the boundary must be fire-resisting, and it can have only very small unprotected areas, such as a tiny window. In practice, a side wall close to the line is solid.
This matters with a timber or SIP kit. The wall build-up near the boundary needs fire-rated boards on the outside, and the supplier should design for it from the start. Tell them how far each wall is from the line before they draw anything.
Gutters, footings and getting round the back
Nothing you build may overhang your neighbour’s land without their agreement, and that includes gutters, roof edges and the spread of a foundation. A wall built on your side of the line with an ordinary gutter often sits a short way in from the boundary for this reason, or uses a concealed gutter.
Think about maintenance too. A wall right on the line can only be painted, repointed or re-clad from next door. You have a legal route to ask for access for necessary work, but it is far easier to agree it at the start.
A simple order of work
Measure the distance from the boundary to each new wall. Decide your roof and eaves heights around the 2 m rule. Give the kit supplier those distances so the walls near the line are designed for fire. Serve party wall notice in good time, ideally after a friendly conversation. Every one of the prefab extension kits we price can be drawn to suit a tight plot, but only if the supplier knows about it before the panels are cut.