Writing off Square Footage on a building for Business

I recently had a friend to ask me whether or not he could write off the square footage of his garage and pool for his Pool/Spa business. And the answer is yes to part of the question. As long as the business taxpayer does not use the garage for personal storage (strictly for business storage) then the entire square footage can be written off. Not only can the exterior (foundation) be written off, but the dwelling as well. This would include any tools used to carryout the business. However,unless you can prove that the pool has only been used as a "demo" you will unable to write this portion off as a business expense. You would hard pressed proving this to the IRS that the pool surroundings were not used for recreational purposes.
Even if you run your business inside of your home, you still can write off the square footage (solely used for business purposes) any 10% of the utilities used to carryout your business on your tax return.