If you liked the architecture profession in the
Sims 3, then you'll like Happy Home Designer ~ especially since you have an unlimited budget (you don't have money at all in the game except for decorating purposes), like Jinglefruit said; in the
Sims 3, you'd sometimes have to cut into your own budget to design your clients's houses. Happy Home Designer appealed to me because of the fact that the gameplay reminded me of the profession in the
Sims 3 (and I freaking miss playing that game, but alas, my computer takes forever to load it, so, no more
Sims 3 for me until I get a better computer
).
You build facilities as well as character's homes as everyone has stated; you can visit the buildings just like you can visit the villagers' homes, but since you have no money in the game (as well as the obvious fact it's not part of the gameplay), you can't shop at the stores you build.
I rarely used (or knew how to make) custom designs in the
Sims 3 and personally, I was never that great at designing houses (plus, I'd be in more of a hurry to play the game rather than try to make a perfect house).
I recommend you to not base your decision whether to get HHD or not, on whether it's like
The Sims or not since they're nothing alike besides the fact you design houses and I believe designing houses in this game is more simplified and limited than it was in
The Sims.