permits, addition, pool

I have renovated a carriage house in Brooklyn and would like to add an addition and possible a pool. The building is 1500 sq. ft. and the lot 5,250 so it is possible, but I am remebering that in the renovation it was difficult ot add a window in the back, because of it being in a landmark area even though it cannot be seen from the street. How do I find out what is possible before approaching an Architect ? I understand that I cannot build within 5 feet of the lot line. Why are the other buildings built wall to wall ? The building next door extends 30 feet beyond mine on the lot line. This is the first time I have posted anything and may be in a totally wrong place, but would appreciate any direction.

might start here:
http://home2.nyc.gov/html/dob/downloads/pdf/swimpools.pdf

then

http://www.nyc.gov/html/dcp/home.html

you can also call up the zoing dept (patience)_ and they can tell you which office can tell you based on your address.

You would also most likely be best off looking for pool builders-  they will know what you can and can't do.