If your doctor says a home pool or spa would help you feel better, you might be able to write it off on your taxes.

In the market for a pool or spa for health reasons and worried about the expense?  Take heart.  The Internal Revenue Service just may subsidize your purchase — if your doctor recommends frequent swims or soaks at home.

A doctor's written recommendation can turn your home pool or spa into a piece of deductible material medical equipment, as long as you satisfy a few IRS conditions.

You will need to install it expressly to help ease a debilitating condition such as arthritis, for yourself or a dependent.  The IRS may want to know about the availability of adequate public facilities near your home.  And the pool will need to be specifically designed and primarily used for medical purposes, not general recreational use (although other people certainly may use the pool or spa, too).

If your project qualifies as a capital-improvement medical deduction, not only can you potentially write off the initial installation costs, but the ongoing maintenance costs as well, year after year.  And this applies equally to renters, as to homeowners.

How much can you deduct?

To determine the amount you can deduct for the purchase of a new pool or spa, begin with the total installation cost, and subtract any aspects of the project that an appraiser might label as having not been medically necessary or just for looks — that imported Italian tile, for example.

Now consider the value of your home before and after the installation.  If the appraised value goes up, the amount you can deduct goes down by that amount.

For example, if your plain Jane pool costs $25,000 to install and this boosts the value of your home by $15,000 you can deduct only $10,000 ($25,000 - $15,000).  If the project has no effect on your home's value, however, you can deduct the full $25,000.  And unlike business improvements, which must be depreciated over several years, you can take the entire deduction at once.

Don't assume you will be able to write-off this deduction in full, however.  The IRS has something like an insurance deductible that it requires you to exceed before you can start deducting medical expenses: the first 7.5 percent of your adjusted gross income.

Let's say your gross adjusted income is $85,000.  Your deduction would need to exceed 7.5 percent of this amount, or $6,375, before you could write off anything at all.  If you were planning on deducting $25,000, that leaves you deducting $18,625 instead.

But since you've now met your deductible, all other medical expenses for the year can be added in at face value, for maximum tax savings.

One last tip: If you do deduct the pool from your taxes now, you will not be able to do so again when you sell the house.  It will not be eligible to figure into the house's tax basis — that is, the cost of the home plus any improvements you have made.

For further information about medical-expense tax deductions, call the IRS at (800) TAX-FORM, and ask for Publication 502