Before a renovation-addition can get underway a plan has to be drawn by retaining a good contractor who can draw acceptable plans required by your local code enforcement. A retainer fee is almost mandatory for a contractor to take the time to measure and draw the building in your contract. A licensed plumber's input is necessary to assure the water lines and drains can reach the existing water feeds and have enough pitch to run drains and vents. Drains need a gravity pitch from the new fixtures to the main drain outlet, or if not, have to be mechanized if possible. With this type of renovation-addition. A "plot plan" or survey of the property lines with the setbacks, has to be available to make sure the addition is within the "building envelope." The building envelope is an imaginary line, front, rear, and sides show the limits you can construct an addition and not encroach on your neighbor. This is vitally important for zoning to approve the addition. If it is beyond the building envelope it will require a variance with the planning board, and all adjourning neighbors ok. If all is within the guidelines you can continue with your plans. Many addition plans are rejected because of inadequate room within the building envelope. If all is ok, the homeowner has to decide what type of foundation they would be satisfied with. A full basement is rare because of the tremendous expense.