How do you build on a lot when you already have a house in sims 3?

To build on a lot in The Sims 3 when you already have a house, enter Build Mode by clicking on the house icon. You can then expand the existing structure by using the wall and floor tools, adding new rooms or modifying existing ones. If you want to change the landscaping or add outdoor features, simply switch to the appropriate tool and make your desired adjustments. Make sure to use the "Move Objects" cheat if you want to place items in unconventional ways.