Resale will depend on time frame as well as the added construction costs and permits.
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1) seek the advice of the building and zoning dept of somerville to see if the zoning, building codes and height restrictions allow it
2) you would need a licensed builder and or structural engineer / architect to determine if the existing ceiling joist meet the code for a floor and can carry the weight of the roof deck.
Any and all new work done would have to meet the current building codes. This will require a buidling permit. Building permits typically require, site plans, floor plans and structural plans. You will also have to have your electrician and plumber pull permits for their work as well.
As to resale it will certainly add to the value and or the marketabilty of the property, but more importantly you will be doing this for your own enjoyment and needs.
Highly unlikely the city will allow a roof deck. Depending on your ceiling clearance in your attic you may be able to expand, youâ€™ll need to check with the building department. If you can do it fairly reasonably itâ€™s probably worth it from an added value perspective. If you make it into say a master bedroom you need to think about another full bath. Just open finished space wonâ€™t add much value. Also remember cooling is very important with third floor space. Definitely talk to the building department before beginning any big plans. Again, if you have any questions donâ€™t hesitate to contact me. Thanks, Joanne