This is kinda how I've always thought of the hideout combined with a little bit of Blaze Big Bang's ideas in his topic here. I agree with him that living in a big house is fine, and makes sense considering how plot-relevant the place is, as well as how many people live there.
The way our Hyrule town developed is that it started with just the market, then grew out a little bit. When large population expansion happened, someone had the idea to drain the (very large) moat to facilitate growth. Some development was done there, but as it worked out, since this was in a previous moat and therefore a less attractive place to live, the poorer of hyrule ended up moving down here. Therefore, the area decayed more and more as the stratification between the haves and have-nots grew. What were previously middle-class type buildings fell into disrepair, and many flimsy ramshackle buildings were erected.

Anyway, about the layout for the Thieves' Hideout: this was once a nicer building, but long ago fell into disrepair. Farin found it abandoned, as many buildings in the slums are. The first floor is mainly a bedroom for the many kids who live here, although a kitchen/eating area is shoved into an octagonal tower space. There are a few shelves with miscellany that the kids have picked up, as well as some cupboards for dishes.
Upstairs, there's a hallway that has kid activities going on, for example a kid might be on the floor drawing on something he found or playing with something, or maybe some kids are playing tag. There's a small study in the octagonal room where Farin has amassed as many books as he can over the years he's been in Hyrule, where he studies to try to find information about Ganon. You can walk over the rafters above the first floor, and if you have the skills, get to something cool at the end. There is also access to the roof by going out one of the windows, or perhaps through a ladder somewhere (this option is not pictured. I just didn't think of it 'till now :p)
I liked Blaze's idea of a secret tunnel under a bed, but I don't know why they'd have that or where it would go.