Self Build Garage with room over in Bath

Hi All Brand new member here. Our old garage finally decided to give up on us and partially collapsed so I've taken it down. This picture from better days. I've stacked the Bath stone on pallets in the driveway (the family is not amused ) and planning to reuse it on the 2 visible elevation of the new building which will be a bit larger. Planning on a concrete raft foundation but before I can really get to that we have a few retaining walls to tak

Thanks. The good news is we have planning permission and I am looking to start in a few weeks. The site is "cleared" and there are a couple of things to cover before we can start on the garage foundation. Yellow line- retaining wall built in stepoc ( hollow block ). Design by structural engineer. Red line – existing retainer/planter. This is keeping me awake at night.😴 It’s basically there to support the ground level on the other side of the stone