Lender answer:

You will need an Assumable mortgage.

Assumption is a process in which one or more new borrowers are added to an exisiting mortgage in place of one or more old borrowers. The lender must approve of the substitution... which will be just like the process of application and approval of the original borrowers.

FHA loans are assumable. Other mortgages can be made assumable if they include a "Novation" clause. "Novation" is a legal term meaning substitution of parties.

You would apply for a loan, you and your SO then when approved you deal with title to the property. YOu might be able to assume your Mom's loan if she set's up the loan that way.