If you want to make your own lots, I have determined that an area can be preflattened by placing a Maxian house from the lot bin either the same size or larger on the spot you plan on building the house. Then just bulldoze it and place the lot, and it will be flat when you load it. I find this is an especially helpful trick when I want to build a new lot in one of the basehoods, but save it so I can use it again somewhere else.
It is recommended that you do this with an existing lot in your game rather than grabbing out of the lot bin. Just because.