Before applying for a barn conversion mortgage, it's generally recommended to get a full structural survey on the barn. This is the most comprehensive type of survey, providing an in-depth analysis of the property's condition and highlighting any potential issues, such as structural problems or dampness. This information can be crucial in planning your conversion and budgeting for any necessary repairs. In addition, the lender will also require a professional valuation to confirm the value of the property before and after the conversion.