Not sure why it would be aappropriate to deprciate a building. depreciation is for things like machinery that will eventually need to be replaced. Buildings genersally dont stop working/fall to the ground after x amount of years and need to buy a new one.
It might be more advisable for the company to have a reserves policy wherby they put x amount aside each year for general maintainence costs etc. but that would be part of their budget meetings after reviewing last years profit & loss.
the mortgage should be put into the accounts as a long term liability, similar to a loan if this helps? the mortgage has nothing to do with depreciation.
please take this in the nicest possible way but it may be advisable to seek help with this as isn't that complicated- are you studying at the moment? it will be covered in your level 3 syllabus if this helps at all.
happy to chat if you need anything.