If you go to another house in live mode and (with testingcheatsenabled true) shift-click on that lot, then "buy on this lot", none of your wishes or opportunities will be affected. But what you buy or build will be charged to your own household funds. Not a problem if you restore it with the familyfunds cheat. In edit town mode you have no obligation to switch households in order to build on other lots. And you'll see no impact on your own household funds. You only need to switch the active household if you wish to play those characters in live mode instead of your own.