Floor Space

The NYC convention is to add up the square footage and add 20%.

This is more like 50% but as long as every apt floor plan in the city misleads consistently I am ok with it! As one can tell I am not from here asking such a basic question.

For condos, the convention in NYC is to measure from the entire thickness of exterior walls to halfway through demising walls. It includes some common elements, e.g. exterior walls, columns, and plumbing/electrical/HVAC stacks. As the disclaimer on most floorplans says, "Stated area exceeds usable area."

The exact language is in the Condo Declaration, the accompanying plans, and the Offering Plan, all of which you'd consult before buying.

you should measure from the outer walls and consider 1/2 of the outer wall yours. if you are looking at a condo, they will add 1/2 the hallway, a part of the elevator, some of the lobby, a little of the pool/excercise area, etc.

look at it from the perspective of does the space work for you and at what price. the $$$ per sq ft is extremely deceiving.

And some of the roof terrace and laundry room?

No. Look at the whole-floor plans filed with the city. Hallways, elevators, stairs, etc., aren't counted.

Each floor has x square feet of unit area, y square feet of common elements, for a total of z. The z's are added up to ensure the building doesn't exceed the permitted floor area.

A few full-floor loft or rowhouse condos will include stairs and hallways, or so people say, but I've never seen an example.