You cannot pass over another owners land without either their permission or an easement. An easement is recorded with the deed and has certain covenants. As far as building an apartment, even if you had driveway access, you would need to seek an approval from the building inspector and have a building permit issued, otherwise, at a later time, they may make you dismantle the entire modification. One other thing, in the event of a fire or catastrofe, you may be help liable for damages a tenant suufers as a result of an illegal unit. (i.e. accidental death due tro a fire, etc)... more
That is a fairly tough question to answer without any specifics as to the size of the rooms you're planning to add, the type of flooring you want, any special windows or skylights, as well as if exterior walls would need to be removed, any landscaping removed where you want to add the rooms, etc.
I would talk to and interview 2 or 3 local builders or contractors to give you estimates as to what you would like to do and then check their references and go from there. They would probably want a plan drawn up of the room additions to be able to give you estimates of the cost to put them on the home.
Good luck with your room addition.
Prudential Prime Properties