How to show variations of a single entry? Even possible?
One option to consider would be to use a structure, with the 'building' as a parent entry and the 'units' as children entries (with corresponding entryType of 'building' or 'unit' to set a unique field layout if needed). 

Based on whatever logic makes most sense you could then display the building, the building with some subset of data from the units (children), or just the unit. The logic could be either an explicit checkbox or switch on the building or unit entries, or just the mere presence of unit entries (children). 

In the structure's settings you can also set whether the 'unit' (child) URI pattern is building/unit (i.e. {parent.uri}/{slug}) or building-unit (i.e. {slug} or {parent.uri}-{slug}).

This should keep everything organized in the CMS and give you a lot of flexibility in terms of what to display where. The building can access the unit data and the unit can access the building data as needed.