I'm trying to turn the entire 57 Road to Nowhere lot into an apartment. It would be a single suite apartment owned by the four Sims from the Singles household. I'm mostly doing this so that they need to work extra hard to pay the rent every week, but don't need an intial "deposit" to buy the actual lot.

My original idea was to put a huge fence around the lot, and add a wall so that I could place the apartment special doors and garage doors (I picture that every household in Strangetown must have a car).

I keep getting an error when trying to convert the lot to apartment base. I suspect it has to do with the huge foundation that holds the house... Are there any workarounds for this? I know about some kind of workaround involving invisible fences, but that one was about the apartment being on a foundation and then leading to a backyard that's ground level. Over here, I have the opposite problem; the "suite" is on ground level, surrounded by a huge fence (maybe that's a problem?), and there's a foundation inside of it.

I've already tried fencing everything instead of having the fence connect to the house, thinking that might've been the problem. Unfortunately, the error's still there.

Now, if it happened to be the fence, sure - I'll place the apartment doors to be on the house and ignore the lot. (Not sure what the Singles ladies would even keep there, but I was interested in charging them more rent.) My next question is how do cars behave if I leave them outside (not in suite)? I don't feel like following split-level tutorials to attach a garage to this house in such case. Can Sims still use it, or is it no-actions-available?

I was able to get this to work, but I had to remove all exterior doors to the house except for one apartment door leading into the house. The garage door was fine. I fenced in the house exactly as you show. From what I can tell, the invisible fencing tricks the game into thinking the structure is entirely enclosed so that it doesn't think there's a second entrance leading outside. Because as far as I know, an apartment can, aside from the garage door apparently, only have one door leading outside (one that allows the sim to go outside and then leave the lot - an enclosed balcony wouldn't count).

Edit: Also, you'll have to fence in the upper exterior exit as well since it also counts as an exit from the house. Also, why in the world does a home this small need three exits on the first floor?

Yes, since the house is on a foundation, no wall or fence that's on the ground is going to have any effect on what's considered part of the apartment. You have to put floor dividers around the external doors like normal, and the garage will be considered part of the common area, so you'll have to buy the car for them before they move in.