I think the challenge of just building on set foundation is enough by itself, but you could add an extra challenge of a budget if you like. Having no budget is less challenging, but allows more creativity, I think.
By the way, this is how mine turned out:
https://cianesimshome.files.wordpress.com/2020/04/screenshot-102.jpg
I was really happy with how the roof turned out, so I built down into the basement to add more bedrooms.
https://cianesimshome.files.wordpress.com/2020/04/screenshot-67-1.jpg
Without hollowing out a basement, it was still a roomy one bed, one-and-a-half bath home.
https://cianesimshome.files.wordpress.com/2020/04/screenshot-95.jpg
My additional challenge was keeping it base game only.
More pics here:
http://cianesims.home.blog/builds/livin-large
