I got it!
I couldn't just check to see if anyone was overlapping the building or not straight away, I had to create some more variables:
1) I created a boolean called "Built" - When the person finishes constructing the building, this boolean will be set to true.
2) I created a variable called Cooldown_Construction_Brand_New with the initial value set to 3 - every second if the value of this is higher than 0, and the boolean "Built" is true, then subtract 1 from this variable.
Once the Cooldown_Construction_Brand_New variable is 0, Person is not overlapping building and building is not a solid, set solid behaviour to Enabled and then regenerate pathfinding obstacle map.
As for the person: I don't need the 8direction behaviour (since setting 8direction down would be bad if the building was against some sort of obstacle in that direction and they would not be able to leave it properly). The person will just walk out of the building in whatever direction they deem possible to get out of with their pathfinding behaviour and 'wander' status that I have set and because the building has not become a solid yet due to the cooldown delay, there are no issues with pathfinding, so after they are no longer overlapping buildings, I can set their collision and solid behaviour back to true (that's what the Still_On_Construction_Site boolean is doing when it goes to false).
Here's a screenshot of my events. Please tell me if you agree with my solution or if there's something better:
Edit: I made some adjustments because I realised that the last event is not going to work at all, haha: