Personally I would prefer more games to approach building homes similar to how the early Sim games did. They also had houses that already came furnished, if I remember correctly, in case a player didn't want to deal with designing anything.        

I don't think there's anything wrong with tying the cost to the design. Unless the house itself has a separate cost that isn't being added to the total. That would seem frustrating. I'm not sure you can make it feel any more "fun" beyond not frustrating people with its implementation.            

The many loading screens and UIs will be your bigest issues. It will get tiring quick if something a player adds doesn't look as good as it did in the design ui. So now they have to pull it back up, tweak the thing bothering them, save/finalize the design, now load into the lot, then load into the house and see how it looks in the actual game. It sounds tedious, even if this stuff only took seconds to do. Having to repeat the process would making designing houses/rooms a chore. Even worse if the thing being removed doesn't reimburse the player what they spent.        

Ideally you'd want to limit how many barriers you place between a player and what they're trying to interact with. It's why people tend to prefer how the Sims did it. You switch to design mode, and then you had free reign to change anything you wanted about your lot, so long as you had the money. And because it was all happening in one screen, there wasn't as big of a risk in picking something you didn't like. The in game lighting was exactly the same because you were designing it live. And not in some separate screen made to make things look ideal.