“How much will it cost?” is one of the first questions we usually hear. Cost depends on many factors from the price of lumber (which can fluctuate greatly) to how busy a contractor is. My twenty years of employment at a design-build firm has given me great insight into how to logically design a structure so that it can be assembled by any manner of workers. Knowledge of construction means and methods, when taken into account during the design process, will necessarily lead to efficient construction. Efficient construction is critical in keeping the cost reasonable.
In general, the size of a structure is often used for initial budgeting, especially for single family dwellings. You will hear numbers such as $200/s.f., or $250/s.f. for estimating. These numbers are very general and can be adjusted up or down in an attempt to take in to account the level of finish, such as whether you are using wood windows or aluminum windows, expensive finishes, high end appliances and plumbing fixtures, etc. But some costs are “fixed”: bathrooms required plumbing and waterproof finishes. Kitchens require plumbing, electrical, mechanical systems, cabinetry and appliances. All these trades are required to some extent, which is why bathrooms and kitchens cost so much more than a simple bedroom addition.
But using the cost/s.f. numbers can’t always come close to being accurate. If you want to add a 150 s.f. bedroom, it’s not likely that it can be done for $30,000. Estimating the cost of a 1500 s.f. addition using the cost/s.f. method is more likely to come closer to reality. The reason for this is that there are mobilization costs, cost of simply getting to the site, and numerous other things that have to be done for every project, no matter what the size. These things are a much smaller percentage of a 1500 s.f. addition than a 150 s.f. addition.