Can you tell us which building you try to access? So that we can look at it as well. If it is not grouped but one object i don't see a chance really. Only if these walls have a separate material. Then you can split the object in it's model parts by materials. If you have the walls separated you of course can simply delete them. What you can try as well is to make the wall material of the original building completely transparent. Then they are gone too. You then have to add some extra walls. Maybe that is easier.

One day your ship comes in - but you're at the airport.