An average price per square metre can be a useful reference, but it quickly becomes insufficient for a period house. Two properties with similar floor areas can have very different values because of architecture, condition, land and the genuine depth of buyer demand.
Start with the property's internal coherence
Layout, natural light, the number of genuinely usable bedrooms and the relationship between the house and its outdoor space all shape buyer perception. A large but awkward property is not valued in the same way as a fluid, immediately usable home.