You may be able to do an initial assessment yourself by looking at the wording of your property title - look in the obligations and restrictions sections. There will probably be an obligation on you to maintain the roof in a good conditions and another one to contribute 50% of any maintenance undertaken. Under 'restrictions' expect to find something about not altering the roof without the agreement of the other parties. If you've got that in YOUR property title, you can be pretty damn sure THEY'VE got the same thing in their title. That means you have a hold over them and they can't do the loft conversion without your agreement. In that situation I know what I'd be looking for - them to pay you a fee for your agreement to this change which was equal to the cost of your solicitor's fees. That way you free yourself of any obligation in the future on the roof for free, and they gain because they are able to enhance the value in their property by extending it.