Answer: There is no legal reason why you cannot buy your parents' house at less than the market value. As you have rightly identified, you have to pay them sufficient to redeem the mortgage. Do remember that with these mortgages, the interest is rolled up — so it is important to obtain an up-to-date figure, which may be more than you expect.
If you do, your parents will have been deemed to have gifted to you the difference between the sale price and the market value and this may have consequences for them if they subsequently apply for state or local authority benefits.
Alternatively, you could purchase an interest in the property and this would be covered in a declaration of trust, which would set out the shares and the parties' obligations. It would be important to agree who would pay the outgoings and for repairs and maintenance in the future, and you and your parents should also update your wills.
Your parents' house is their main asset and it is important that the family takes advice from a solicitor specialising in this area before a final decision is made.
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If you have a question for Fiona McNulty, email firstname.lastname@example.org. We regret that questions cannot be answered individually.
Fiona is a partner in the residential real estate team at Thring LLP (www.thrings.com).
These answers can only be a very brief commentary on the issues raised and should not be relied on as legal advice. No liability is accepted for such reliance. If you have similar issues, you should obtain advice from a solicitor.