Taking Worthy's comment one step further, you might be able to do the math by taking the overall sqft cost and deduct the non-changing majors (HVAC, windows, doors, appliances). The figure you have left can then be divided to determine the adjusted sqft cost. As an example, 2000 sqft @$105 per equals overall cost of $210,000. Taking figures out of the air to cost HVAC, windows & doors (low priced), appliances, mediocre roofing shingles, and tiling for kitchen and bath, I came up with a majors cost of $40K which was then deducted from the $210K; that gave an adjusted sqft price of $85. That means removing 250 sf from the original 2000 would save you about 10% of the dollars while losing 12½% of the space. Personally, I think that's a bad deal. It is far more likely that you can save an equivalent amount (or more) by buying the least expensive for all replaceables. "Replaceables" means anything which is not actually structural, and which can be replaced without excessive effort. That means all appliances, kitchen and bath counters and cabinets, flooring, and doors. i.e. that lovely $2K front door should be put on hold and a $300 one used instead (or refinish a discarded antique for even less cash out of wallet) and using vinyl tiles instead of terrazzo. Some may disagree, but I consider roofing to be a replaceable -- unless you are spending big bucks for tile or metal, roofing IS going to be replaced at some point. Therefore it is not unreasonable to use elcheapo-15 year warranty now and in five or ten years replace with the wonderful bigbucks-50 year warranty stuff. The difference in cost is considerable. Unless you are planning to use insulative layers of window covering [$$], it is *not* cost efficient to not use sealed double-pane glass. However, using standard sized ready-made windows instead of custom sizes can save quite a bit (multiplied by the number of windows). Using elcheapo hollow-core interior doors can also provide a nice savings. Some savings can be less obvious, such as standard chrome faucets from a box store (the price difference can easily be $100 per, times the number of sinks). It can be painful giving up "I-wants" for the new house, but if you think of the less expensive stuff as temporary and the wanted stuff as deferred pleasures, it can be bit easier to accept the differences.

You measure the exterior. Garages, porches, 3 season rooms, decks, lanais, basements and attics don't count (attics count IF finished, are walk-up, heated and have windows). Basements don't count at all except as an add-on, such as "3000 sf colonial with an additional 1500 sf finished basement". Measuring the exterior sounds easy, but it isn't unless the house is basically a box. In my case, I had to account for multiple bump-outs and a room that is really only on one floor but from the exterior appears to have a room above it. There are formulas for when there are slanted walls, such as knee-walls which Capes have. I know I found it online once when I searched for it once. The ceilings have to be at least a certain height (7'6" maybe?) and the knee wall needs to be 4', and part of the square footage is subtracted to account for the square footage from the knee wall to the minimum height requirement. The real estate appraiser we used and the town both said the house was 4200-4300 sf. It is actually just over 4000. I am far more confident of my measurements than theirs. I am fairly certain the town double-counted the great room (which is 2 story, so no room above it even though it appears that way from outside) and the appraiser sort of rounded up the measurements (house is 32' deep at some spots and 36' at others, he just used 36'). Now, room sizes are different. Listings will often give square footage and room sizes. Room sizes are based on interior dimensions, not exterior.

Is the ceiling at least 7'? Is the width at least 7' with a minimum of 70 sq ft of floor space? Does the room have operable windows that can provide light and ventilation, and are large enough for egress if the room is to be used as a bedroom? Are the walls insulated to the current code required R value for your location? Is the ceiling also up to current standards of insulation? Is the heating source a permanently affixed non portable type? Can it heat the room to at least 65 degrees? Do each of the walls contain the required number of tamper resistant arc fault electrical outlets that are required by the current electrical code? Did you get the permit and the inspections needed in order to convert the space? Without all of that, what you have is a utility room with a space heater that no one is dumb enough to think of as living space.

The *percentage* of house that is kitchen will vary depending on the size of the house -- and what you consider 'kitchen'. Our house 'lives' single-level -- for us. Finished stairs lead to a full-height attic that can become a second level with the addition of dormers -- for some future owner. Our 2900 sq ft home can become a 6000 sq ft home -- rooms over our finished space and over our attached garage. We built this way because our acre+ is zoned for the larger house, and most of the neighboring homes are at least that size. The 'working' portion of my kitchen is 320 sq ft. Within the room are also a desk and large buffet -- 'kitchen' becomes 370 sq ft. At one end is an eating area -- another 150 sq ft. Between kitchen and garage is a back hall -- 150 sq ft. Include what you will and get different percentages for the house as it is and as it may become. Texas_Gem -- In my area only above-ground HVAC-ed space is counted -- and taxed; no basements, finished or unfinished -- and no attics.