It is one of the title deeds. Other title deeds would include the Property Registration Documents (eg Folio), Family Home Declarations, mortgage deeds, planning documents, deeds of rights of way etc. If the house is subject to a mortgage, the bank should/would have the deeds. A good starting point is with the owner’s solicitor who handled the house purchase/mortgage, as some solicitors would hold a client’s deeds in storage for safekeeping.