how easy is it to obtain a certificate of lawfulness?

i have to show that the garage behind my house has been used as a commercial garage from july 1948 and that the period at the end where it wasnt was not intended to be permanent.it is now currently in use as a garage and i have found evidence going back to 1950. with ';hearsay'; about 1948-50.i understand this bit of paper could increase the value of the garage, by how much?does hearsay count.?