Because you are considering adding a garage, your best bet is to go to the building inspector and explain your idea, he will school you in municipal regulations and exactly what you will need to do to get your building permit. This will include a review of the size of the lot, the required setbacks and ability of the land to withstand such a construction. It will also inform you of the requirements that Nashua has for building a garage--including required wiring, egress door and needed curbcut to the road. All this information will be free right down at town hall.
If cost is no issue, find a good contractor, tell him what you want and let him get the permit and do the work for you.
Good luck with your project