Rear and side extensions and party walls

Building on the boundary

Building right on the boundary gives you the most internal space, and you have two choices:

• Astride the boundary. The wall straddles the line, but only with your neighbour's written consent. Costs can be shared if they later use it.
• Wholly on your land, up against the line. No consent needed for the position, though notice is still required. Where necessary, footings can project under the neighbour's land from one month to a year after the notice, at your cost and with compensation for any damage.

Building against a neighbour's existing extension

If next door already has an extension along the boundary, you may want to build against it. Cutting into it, bearing on it or enclosing it raises party wall questions, and your design should address how the two will meet and be weatherproofed.

Foundations

Extension foundations near a neighbour almost always trigger the 3-metre rule. Where the neighbour's house or extension has shallow footings, the award may require sections of your trench to be dug in short lengths, or other precautions.

Flashings and gutters

Where your new roof meets the neighbour's wall, a flashing is usually cut into it. That is party structure work. Gutters overhanging the boundary need the neighbour's agreement, so design them to sit on your side.

Practical points

• Ask your designer to show the exact boundary position on the plans.
• Think about how your scaffolding will be erected; if it needs to go on the neighbour's land, access needs to be arranged.
• A good relationship at the start makes consent to an earlier start more likely.