if you can answer yes to the regular use question below, it would seem you should have gotten the HO deduction. we have no access to your return so you may need to revisit the section:
is the area used for business used on a continuing basis?
Occasional or incidental business use does not meet the regular use test, even if the area is used for no other purpose.
it may also be that you have run out of deductible HO expenses. after 40 years the office would have been fully depreciated and it's kind of doubtful that you had a mortgage interest deduction. taxes would have remained deductible. the deduction for other expenses could have been limited.
