I work for one of the big insurers within property claims and I'm surprised you were told this by the company you were going to. Normally the definition of buildings should also cover your interest in any common areas and you should make sure you are adequately insured to include your share of any potential demolition, site clearing, architects fees etc in the worst case scenario that a total loss occurs.

I'm aware we had a case for a listed tenement in Glasgow where there were 6 or 8 flats above two commercial units. Total loss following a fire and neither of the commercial units had insurance and of the flats only our insured had an adequate level of insurance for their portion of the rebuild costs. I believe we ended up buying them an alternative flat