A heat network uses a shared source to provide heating, cooling or hot water to more than one property. Where shared plant serves several properties inside one building it is a communal heat network. Where it supplies more than one building it is a district heat network.
Do not assume one building means one heat network. A building can contain more than one. A single network can serve several buildings. For every network establish what it serves, who receives heat, who operates it, who supplies it and whether more than one organisation is involved.
Having shared heating does not automatically mean every arrangement is regulated in the same way. Ofgem identifies specific arrangements that can sit outside the framework. Check the individual network rather than assuming it falls one side of the line.