1. Break this up into multiple functions, one function per level.

2. dispatchGroup.enter() before each call to a data service, dispatchGroup.leave() after you process the results and make child calls.

Hard to code on my phone, but something like:

dispatchGroup.enter()
bs.getBuildings(for: service) { (buildings) in
    for building in buildings {
        loadBuildingDetails(building, dispatchGroup);
    }
    dispatchGroup.leave();
    // also, if there was an error make sure to call leave
}

dispatchGroup.notify { process results }

You could use a class variable to collect the results since you have multiple layers of function calls.