@Evalen You just have to have the value of 350.000 $ in the lot, it has not to be in the real house, just put a wall and stuck content on it so you can delete it once the goal is reached, you can use MOO and stuck content there windows, paintings, pillars, anything, it will count the same for the lot value.
In real life, I work with architects, we had a client who sold his home to be able to build a new and better one and he lived in a tent at the garden while we were building it (because he got moneyless), your idea brought this back to me, lol!