"Christian McArdle" wrote
The neighbour pointed out the pitched roof and gutter of the extension
would be over his garden.
So the plans and footings had to altered and there was significant extra
expense incurred.

Yes, it is prudent to ascertain that you own the land before you design and
build!

Or air in this case.

Is it a daft question to ask that the boundary extends 'virtually'
upwards from the property line. How far ?

Can I charge BAA for entering the airspace I own above my house ;-)

P.