Under the Party Wall Act, where should my extension wall start or how much space should be left?

This is something that needs sorting out at the planning stages, there are instances of building flush with dividing house, and then for the neighbour to complain that facias boards and gutters are over hanging their air space. You may have to step in 100/150mm. One of those things to agree before starting building, your architect should have advised and shown all dimensions on the planning drawings.

Providing no part of your building encroaches upon your next door neighbour’s property, this includes air space then you can build right on the edge of the boundary. To prevent your roof from over sailing into your neighbour’s air space you could always build a parapet on one side of the building. All this should be sorted out in the planning stage and any good architect will be able to provide you with answers to any question you have about your project. If he can’t then is he the right one.