You'll see an empty square, those are the only "new" lots you can build one, for the OP you CAN move a char onto the empty lot, build the house (which is what I do for starters so I -know- it will be within price range ^_^) and when you're done building use their cell phone to use the move interaction. Sell the furniture when you move (otherwise they'll take it all nasty thieving sims!) then you can go into edit town and click on the empty house it'll tell you about how much it's going to cost and you can add a few extra things before you share it with us all
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Then go to the build tab and ta da!