Effy,
You can put up a building on your own property and take it as an expense over the life of the building (39 years) and even lease the land under the building to your business. In your case you have legitimate business purpose so you can take the expense. If you have your rental business in a corporation then you would lease the building from yourself. If you are in an LLC or sole proprietorship then you would take the business expense of depreciation directly on your Schedule C. The only downside is that part of the sale proceeds of your principal residence will now be subject to tax, but my guess is that the current deduction is more important.
Ed