Posted: Sat Mar 6, 2021 11:16am
The problem is going to be whether the Ajuntamiento in principle will permit it, a visit to them and a discussion with the Architect will clarify.
If the existing ceiling height does not comply with building code requirements, you may face having the floor dug up and lowered, I think the minimum habitable ceiling height permissible is 2.5 metres.
If it is completely underground, then lighting will be a problem as will ventilation, but if you have a wall where windows could be installed that will help. For what purpose do you want/ need the conversion? Bedroom? Study? Damp could be a problem ,but you would need the area completely sealed against any ingress of water.I would schedule an appointment with the Town Hall Architect and take details of what you would like to do, in the first instance, to see if it would be allowed. You can then employ an Architect to draw up plans for you as this will require the correct licence. The conversion will have to be added to the Escritura as it increases the floor area of the property.