This is a great question, and one that is very important for pastors to understand. In a nutshell, any expense that is used for the upkeep, maintenance, or provision of your home may be counted towards your housing allowance. This includes:

Rent, principal payments, down payments, etc.
Taxes, interest, and insurance on the home and its contents
Utilities
Expenses related to purchasing furniture, appliances, decorations, and household goods (e.g. sheets, dishes, towels, etc.)
Improvements
Repairs
Miscellaneous items (e.g. snow removal, lawn mowing expenses, light bulbs, cleaning supplies, etc.)

As you can see, that is a fairly extensive list!