I generally like the idea of being able to have a separate industrial area if I'm trying to make a larger town but as we have seen, the further away a citizen lives, the less productive the building will be. So the idea of having a family live next to or inside the same building is a definite solution and I believe it was you <a contenteditable="false" data-ipshover="" data-ipshover-target="<___base_url___>/index.php?/profile/3183-ketchup/&do=hovercard" data-mentionid="3183" href="<___base_url___>/index.php?/profile/3183-ketchup/">@
Ketchup</a> that had some mods using this idea, e.g. the tailor with an add-on house so we know that it is possible to make something like this.
However as mentioned by <a contenteditable="false" data-ipshover="" data-ipshover-target="<___base_url___>/index.php?/profile/3-kralyerg/&do=hovercard" data-mentionid="3" href="<___base_url___>/index.php?/profile/3-kralyerg/">@
Kralyerg</a>, <a contenteditable="false" data-ipshover="" data-ipshover-target="<___base_url___>/index.php?/profile/5518-necora/&do=hovercard" data-mentionid="5518" href="<___base_url___>/index.php?/profile/5518-necora/">@
Necora</a> and <a contenteditable="false" data-ipshover="" data-ipshover-target="<___base_url___>/index.php?/profile/3183-ketchup/&do=hovercard" data-mentionid="3183" href="<___base_url___>/index.php?/profile/3183-ketchup/">@
Ketchup</a>, it has to be one family only per building so a possible solution would be to do as Ketchup has done and also as can be found in <abbr title="Colonial Charter">CC</abbr> with the Themed sets (there's a bakery that can have another two floors added above it, each floor is a separate house). As Necora mentioned, it could be modular, exactly like what Ketchup has done with his Mexican Fantasy mod and Necora has done with the Port Royal addition to his Maritimes mod.
I know I am just basically repeating what the rest of you have already said but I'm thinking of how it could be implemented... so for example, the ground floor could be a large sized production building and then the second floor could be multiple modules so that two or three (or maybe more depending on how big the base building is) houses could make up the second floor (just like a set of apartments above a ground floor shop). The houses would lock to each other in the same manner as Ketchup's Small Town Row Houses mod. To take the idea further a third floor could be made using the same technique as the second floor. In this way you could have one production building with several homes directly attached to it so that it looks like one large sized, completely industrial looking building.
In this way, you would also get the ability to make each building look a little different, for example, one production building might have just one floor of homes above it while the next production building has two floors of homes above it. Even the number of homes per level could be different with say two homes above one production building and four homes above another production building. I wish I could find an image of what I'm talking about but if necessary I'll draw one free-hand and post it if it would help make it clearer.