Another aspect of your project that can add to the cost is the complexity of the design, the linear footage of new interior walls, the number and complexity of cabinets, finish trim, etc. Obviously, it would be cheaper to have one, large, unobstructed space, few interior walls, fewer cabinets, etc., than something of greater complexity. Also, most commercial spaces tend to be simple in their perimeter space (basically just a big box) and all on one level, but if you have a space with a more complex shape or multiple levels, this will definitely add to overall per square foot costs.

Site Improvements-

For projects in new buildings, most of this should already be taken care of with the original permit, but do keep in mind that as spaces fill up, the local jurisdiction tries to keep track of parking requirements and if your are one of the later permits pulled on the project, may actually have to account for extra parking, this can be quite expensive.

In projects in older buildings, you will often be required to insure that adequate handicapped parking is available onsite, as well as other handicap accommodations on the exterior of the building such as ramps, signage, etc. Other items such as new trash enclosures and other site improvements may be required.  Be sure to check with your local jurisdiction early on these issues.

Back & Forth Some More-

So, as I said before, you can keep narrowing down your construction costs by going back and forth between the estimate, and further refining the detail in the plans.  You can start picking out very specific finish materials, fixtures, etc., and get things right down to the dollar for  those types of things.  It is a good idea to get a contractor involved early in this process, as he may have further insights into current costs which are changing all the time.  Generally, contractors will provide estimates free of charge, especially if they think they have a good chance of landing the job.

Price vs. Value-

If you are unfamiliar with construction costs, it can be a good idea to get several estimates or bids from different contractors.  But the lowest price is not always the best value.  Time is money so as you are paying your monthly lease payments during construction and are not open and in business generating revenue,  a more experienced, possibly more expensive contractor may actually save you money in the long run.  Experience will also pay off in the quality of work, and a reduced need to call that person back to fix things that weren't done right the first time.

Online & Printed Data-

Cost estimating books like RS Means have been around for years and can give useful per sq.ft. costs broken down by various project sizes and types.  And now they even have online estimating tools.  These can be a great place to start, but I will caution you that the construction market, especially in the residential area, has been historically quite volatile.  So trying to compile accurate figures and get them published often means they are behind the ball.  Also, if you are in a smaller community, local market forces (next page) may mean significant differences from the figures provided by these types of sources.

It can be sort of considered similar to predicting a recession.  You only know after the fact, sometimes by many months, whether one has started or ended.  And in the end, the price isn't set until you find an actual person to agree to do the work for a certain price, and then not often until the job is finished do you know the actual cost.