If you have a mortgage, does that mean its on the property too or just the building?

A mortgage on real estate generally includes the lot, unless it's a condo or a townhouse. This will be spelled out in the property deed. What you are suggesting is actually refinancing the existing loan and adding the modular unit to it. Talk to a mortgage loan officer at your bank or savings and loan. It's possible that you can do this, since your mortgage balance is so small. Your credit score will have a bearing on the bank's willingness to do this, but the equity you have in the house you are currently living in may be sufficient collateral to persuade them to do it. You've gotta talk to the lending institution to determine the feasibility of what you want to do.

it depends on how it was written

it usually is on both when it comes to people buying homes to live in

but again if this is a spacial case and the land is owned by an association the house is on..then it very well be on only the house.