Neighbour wants to use my planned extension to build double story extension.

[Norman29]

Hello, I live in a house that has been converted into 4 flats, the owner of each of the flats has a share of the freehold. My lease includes my flat and the full length of the garden outside of my flat. I have recently submitted an application for a single story extension and my neighbour above has requested if I would consider submitting a second application for a double story extension instead. My architect has said this is high risk for a number of reasons and our other neighbour has said he would refuse to sign this off but the upstairs neighbour is still very keen to use my extension in some way - perhaps a roof terrace. What I would like to understand is how her getting access to build above my extension would affect our leases and would she need to compensate me is some way for using land that falls within my lease to increase her floor space?

[Interlaken]

Surely if 4 of you own a share of the freehold then you all have to agree to the side extensions as the freeholder will own the walls of the existing building?

[Lawcruncher]

Your lease includes the garden. Unless the lease provides otherwise, this will include all the airspace over the ground. Accordingly, you have complete control over the airspace which will be above the proposed extension.