Keeping a building in an usable shape, that takes money.
On BM, all the buildings except for player houses and the townhall require a maintenance fee, per month (ingame month, which is 1 real week).
If this fee is not paid for 3 months, the building becomes disabled. Once in the disabled state, you can't use it anymore, until you pay off the debt, but it takes less money to mentain it. 10 months of unpaid debt, and poof, the building is gone.
The player houses will be taxed by the mayor (he or she can set the city tax), and if the player doesn't pay for a certain period (to be determined) the house will become the property of the city.
The mayor will be able to pay the maintenance cost of the buildings as he or she sees fit. For example, in case there is a lack of funds, the mayor can pay only for the really used buildings, and pay for the others when there is some money available. Or just let them decay.
Today I wrote the function that checks every month for maintenance costs, and for buildings being disabled or destroyed.
Tomorrow I will start writting the City Secretary NPC.