It all depends if are allowed by town's zoning regulations to add the basement to your living area or not.
If permitted and you can added to your public record and eventually to your deed, then you have a golden egg here to hatch. In the other hand if by zoning law you are not allowed to do it, then you may have an finished space which may appeal to some buyers, as bonus, but not willing to pay a much higher price for it, and you would not be able to listed on your marketing material as extra square feet of space.
I don't know the size of your basement and the level of finish you are planing to do, but do not over spend.
I suggest you consult an attorney regarding the zoning regulations, you may be able to get variance.
Please let me know if i can help.
If you are planning to sell rather soon you really need to compare your unit to others on the market and recently sold statistics, and estimate what your listing price (and potential sold price) might be with or without finished basement. It is rather a case by case basis that is dictated by current market conditions.
I would be happy to do a free Comparative market analyses (CMA) for you.
Even to a buyer, "Finished" space in the basement will not be worth as much as the equivalent space above ground. It is always good to make basement space clean and light - perhaps painted in light colors instead of industrial gray, but don't spend too much money that you won't get a return for.
It also makes a difference whether the space has direct access out side and how much natural light it gets.
For a basement space to count as a bedroom, according to the building department, it must have a window within 4' of the floor that is unobstructed and large enough for a person to get out in case of fire.
In short - some basement spaces are more appealing and desirable than others and use a very conservative estimate of what it will add to your condo's value.
It is always best to have a professional look at the place before doing anything (or not)! If you need assistance with your space specific suggestion, don't hesitate to contact me at firstname.lastname@example.org. I'm also a Brookline resident.
I am here to help, and would love to assist you with marketing of your home. PLease contact me with questions.
Kenzo P. Tatsuno
Coldwell Banker Residential Brokerage