Much will depend on the actual plans, extensions, etc., work with a good licensed local contractor, as permits are needed--your contractor should be able to help you in estimating new applicable taxes--or if you already have a plan in mind contact your local tax department and inquire.
The way I would do this is to ask somebody in the know there to give you an informal guestimate. The square footage, number of bedrooms and baths, and some other factors (fireplace?) will also affect your tax burden. So have some details decided upon before you approach them. Also, ask them for the name and contact number of someone knowledgeable in the property tax office.
Remember that one purpose of building inspections and permits, in addition to safety, is to make sure that the property is appropriately taxed.
Karla Harby, VP
Rutenberg Realty, Manhattan