Can I build an extension right up to the boundary in London?

On the line or just inside it: what the Party Wall Act decides

If you want the new wall to sit on the line of junction itself, half on your land and half on your neighbour's, section 1 of the Party Wall etc. Act 1996 makes you serve notice at least one month before he intends the building work to start. The neighbour then has a choice. If they consent by notice within 14 days, the wall is built astride the line as a party wall, and the cost is shared according to the use each owner makes of it. If they do not consent within the period of 14 days, you may still build, but only at your own expense and wholly on your own land.

Many owners skip the astride option and go straight for a wall wholly on their own land, with its outer face on the line. That still needs a section 1 notice a month ahead. The useful part is section 1(6): with that notice, you may place projecting footings and foundations under the neighbour's land where they are needed to build the wall, at any time from one month to twelve months after the notice is served. You pay for that work and for any damage it causes.

The Act stops at the footings. Nothing in section 1 lets gutters, eaves, fascias or render overhang the neighbour's side above ground. On a wall built tight to the line, the roof usually has to drain inwards: a parapet with a concealed box gutter, or a roof that falls away from the boundary. A rainwater pipe or trim that strays 50 mm over the line is enough to start a dispute after the work is finished. We draw the boundary detail at large scale so it is fixed before the bricklayer arrives.

Digging foundations near the neighbour's house

A separate notice covers the trench, under section 6. It applies where you excavate within three metres measured horizontally from any part of a building or structure of an adjoining owner and go deeper than the bottom of their foundations. It also applies within six metres where the dig would cut a line drawn down at 45 degrees from the bottom of their foundations. The notice goes out at least one month before you start to excavate.

On a London semi or terrace, a new rear extension is almost always within 3 m of the neighbour's own back addition, and modern trench-fill footings on London clay are often deeper than the shallow Victorian footings next door. So expect section 6 to apply even when the new wall stays a little inside your boundary. The notice needs drawings showing the depth and position of the dig, and those come from the same set as the planning drawings. What each notice costs, and when surveyors get involved, is in our guide to party wall notice costs; our party wall notices and drawings service prepares and serves them.

Fire rules for a wall close to the boundary

Building control looks at the same wall from a different angle. Approved Document B Volume 1 measures fire separation to the relevant boundary, which for a house is normally the site boundary. Paragraph 11.8 says an external wall on, or within 1000 mm of, the relevant boundary should be fire resisting from both sides, with its external surface class B-s3, d2 or better. Brick and block meet that easily. Timber cladding facing the neighbour usually does not, which is why a clad garden-side wall often turns back to brick along the boundary.

Openings are the bigger constraint. Within that 1000 mm, the only unprotected areas allowed are the small ones in Diagram 11.5, each no more than 1 m² and spaced apart. A side window in a boundary wall has to fit those limits, so a run of glazing facing the fence is not an option. The fire boundary is the legal line, not wherever a hedge or an old fence happens to have grown or been put up. A hedge that has crept 600 mm onto your side does not move the boundary, so the wall has to be specified from the measured line. The building regulations drawings carry this specification for building control.

Where is the boundary, really?

Most owners take the line from the Land Registry title plan, and that plan was never meant to be precise. Section 60 of the Land Registration Act 2002 says the boundary shown for the register is a general boundary, and that a general boundary does not determine the exact line of the boundary. A plan drawn at that scale cannot show whether a fence sits on the line or 100 mm to one side of it, and 100 mm is the difference between a wall on your land and a wall on your neighbour's.

So the position of the fence, the title plan and the deeds can all disagree. Before anything is drawn on the line, we survey the boundary features as they stand, the fence posts, the old garden wall and the neighbour's back wall, and set them against the title plan and any older conveyance plans you hold. Where they conflict, the cheaper course is almost always to agree the line with the neighbour in writing first, or to set the extension a little inside it, than to argue about it with the foundations open. A measured building survey gives that baseline.

The side of the house raises the same question in a different form, because a side extension on a semi or end-of-terrace often runs the full length of the shared boundary. Our guide to the drawings a single-storey side extension needs covers that layout.