Simply click the Lots button on the UCP, then select the Lots & Houses bin.
If you're not interested in building houses, the game ships with a few empty houses ready to put down. We'll talk about advanced strategies with house building later. You'll start with 20,000 Simoleons, and trust us, that's not too much. Because of that, you need to be very careful with what you build.
So, if you go tree or flower happy, you'll be shelling out huge cash for the same house. That said, flora now contributes to the value of the house. And that, trust us, is what you're going after. By building a house beforehand, it loses some of its value (and therefore its price) before your sim gets a hold of it. Walls are expensive, and trying to build a house will completely destroy your budget.
The reason is because of the automatic depreciation of the value of the house. That is, you build your house however you want before moving a Sim in, instead of moving the Sim into an empty lot and building a house then. Like in the first game, it's cheaper in the long run to 'pre-build' a house.