If by apartment you mean separate and self-sufficient living quarters then you would have to check with your local zoning board because some towns allow an arrangement with all fixtures except for a cooking stove – others may go so far as to limit the number of sinks! This would also pertain to a finished space, be it attached or detached above a garage. Also, most towns limit the number of bedrooms by the size of the lot, regardless of whether they are in the main structure, above or below grade or in a detached structure. As for below grade living spaces (i.e.; apartment or bedroom) being sanctioned as legal, here in Massachusetts direct egress from each bedroom is essential. Therefore, a minimum of one 4'x4' window in each bedroom is mandatory.

Scott is correct. It is important to speak with the local authority. If you are looking in Oak Bluffs the link to the building department is below. The by-laws are posted there as well.

There are multiple conditions that need to be met. Including ceiling height, means of egress and other building codes. The best answers can be recieved from your local inspectional services/building department in your local municipality. There are often special conditions in every home that will need to be addressed that are specific to each home and any plan would need to be reviewed by the city/town and done by licenced contractors and/or done to current building codes.

Good luck, hope that helps,