Most likely you will need a permit, stop by at your building department and ask.

As the construction goes it's a simple project and as the cost goes get a few estimates from local contractors because the prices vary between regions, states, countries, and contractors who do the work so your best bet is to get a price from someone local.

(not a pro) Also if this is a condo or townhome (rather than a single-family home or other building you own entirely) you will probably need approval from the condo board / HOA.