Sale of commercial property owned personally by the director

A director has personally owned the unit which his company has traded from for a number of years and received a rent each year.

10 years ago the unit was extended and the company paid for all the work.

(No buildings capital allowances have been claimed in the company books for this extension)

Q-If the director now sells the property and a gain realised then how is the building extension to be taken out of the company's books dealt with? as in reality the company has paid for part of the building and not the director-do you just pro rata the proceeds?

Thank you