Neighbour's Extension, Not convenient For Me, Can I force them to Change Extension Plans ?

Hello Not knowing the whole picture I can informally comment as follows; I assume you share the building with your neighbours and as such the property is not classed as a single dwelling house, in planning terms. In response to house doctors answer, both you and your neighbours do not have the benefit or permitted development rights. Even if the extension were classed as permitted development one should apply for a Lawful Development Certificate for proposed use. However, assuming I have interpreted your position correctly, should they decide to submit a planning application then they would need to submit a full planning application and all neighbours [ and the freeholder/s ] , including yourself would be consulted as part of the planning process. Ideally, if you wanted to use their roof in a future planning application then you would need their agreement. Most ground floor rear extensions we design include rooflights so that would restrict you extending your flat over their extension. You should submit a joint application with them including both extensions which is more likely to achieve planning permission, however they may not want you extending over their extension. It would be difficult for you to object to their extension, for valid reason as I expect their extension would have minimal impact on you ? Regards Pete

If there extension falls within permitted development then you may struggle to stop them building what they want as they will not need to go through the planning procedure. I am not sure but with flats there must be slightly different rules to house for extensions..... I am not even sure if you would be allowed to build on top. Call your local planning office at the council they can help

Put simply - talk to them Explain what you are thinking about doing, if so then offer to pay for half the costs, etc. However they are under no obligation to agree and you cannot force them to do anything! Hope this helps paul