Why did you replace them and where is the dog boarding occurring?
If clients are having to walk up and down the steps, and you have an area in your home that you use for dog boarding, then you would be able to take a prorated deduction. For example, if you use your entire basement which is 2000 sq. ft. exclusively and regularly for dog boarding and your total house is 4,000 sq. ft. including the basement you would be able to deduct half of the $15,000 as an asset (not an expense) for the dog boarding business.