It's not 10% of the value. It's 10% of the policy amount. If you have a $200,000 policy on the house, $20,000 of that can be allocated to the garage. You'll probably have an issue with the examiners, though, if it's a really nice garage with a RCV of say, $40,000.
The second building needs a separate policy.
I thought getting old would take longer.