It appears that the gist of the question is this - Is a loan to purchase and place a shed on a piece of property where there is a dwelling considered a home improvement loan under HMDA.
Since the definition of home improvement both now and in 2018 and beyond include both improvements to the dwelling and improvements to the real property on which the dwelling is located, this would meet that standard. Beyond that, there is now a distinction between 2017 and 2018 and beyond.
For 2017, in order to be reportable as a home improvement loan, the loan to purchase the shed must either be secured by a dwelling OR classified on your books as a Home Improvement Loan if not secured by a dwelling. If the loan is not secured by a dwelling and it is not classified on your books as a Home Improvement Loan, it is not HMDA reportable.
For 2018 and beyond, the loan will only be reportable as a HMDA loan if it is secured by a dwelling. If the loan is not secured by a dwelling, then it will not be HMDA reportable.