My neighbour is building a brick garage on the side of his house and the external wall will be 15cm (fifteen cm) from the boundary line. The plans state that the foundations will go up to the boundary line. The building will create a small alleyway between his and our side walls. There is exactly enough room up to the boundary line on our side for us to take our black wheelie bin down the passage.
We intend to gate the alleyway for extra security but this leaves a gap on his side of the boundary line of 15cm. He intends to run his downspouts for gutter water onto the outside of his external side wall into the alley and into a grid presumably. Can he do this with only 15cm knowing that he will have no access to his downspouts or grids? His proposed garage extension allows him no access to his back garden as he has left only a strip of 15cm (presumably for overhang of gutters and soffits).
An owner can build right up to their boundaries including the airspace above their land. This will not, however, provide your neighbour with a right of access in future to maintain rainwater fittings etc. If such access were required, and you were unwilling to grant it your neighbour would have to obtain a court order under the access to Neighbouring Land Act 1992.
There is also the issue of access to build the garage – if the work does not come within the scope of the Party Wall Act access would only be with your consent and the Access to Neighbouring Land Act cannot be used for new work; only maintenance. The work will come within the scope of the Party Wall Act if the foundations for the garage are within 3 metres of your property and deeper than your foundations. If that is the case your neighbour should serve you with a Notice of Adjacent Excavation.