Family owns a house with an adjacent lot. They are wanting to sell the house want to keep the adjacent lot. What do they need to do to separate the empty lot from the house? Thanks for the help

If this is a subdivision, the lots are likely already separate. Only way they wouldn't be is if the original plat was changed to combine them. If that is the case, the plat needs to be changed back. I know an engineer can do a replat, but a good surveyor may be able to do it for cheaper.

If it is two separate, legally defined lots per a subdivision plat, it's pretty easy to do. You just sell the adjacent lot. But you need to be absolutely sure they are both legal lots, not just what the CAD says. It would be even better if you can confirm if the adjacent lot doesn't lose some kind of grandfathering that may or may not be in place. You will also want to check the location of utility hookups to ensure everything is on the lot with the house.

is there a mortgage on the combined parcels?

I believe so

Assuming the mortgage is across both legal descriptions you will not be able to seperate the lots until the loan is paid or the lender gives a partial release, releasing the lot as collateral. That is very unlikely to an option assuming the mortgage is not held and serviced by a small local bank. But, even then it would result in principal paydown etc as the lot value was included in the orginal loan and loan to value etc.

The way to do if from the mortgage perspective is refi the house, removing the lot from the financing, which would then allow you to sell the house and keep the lot as it is no longer encumbered.