The monthly income by each house or apartment is strongly influenced by the happiness of its inhabitants. Demolish buildings near to them only when you also have the money to clear the plot of the debris immediately afterwards to make sure their happiness does not decrease.

The happiness of a family house is increased by one for each family house that is built next to it. The happiness of apartment blocks are not affected by other apartment blocks. Note: The houses' happiness will decrease when they are built near old blocks, old houses, apartment blocks as well as the post office, the supermarket and the restaurant, while apartment blocks are not affected by those buildings.

While houses and apartments are upgraded or get repaired you will not get a monthly income from them. Upgrade and repair when you just received the income to minimize this.