Let's go with my example, I have 3 tables and I want to know what's the best method to name my resource url.
You first need to figure our the requirements for your API, the scenarios, use cases etc, and only then actually start building it.
To go back to your question, I'd go for
GET someapi.com/buildingTypes, because
/buildings/types will leave you with a "Types" resource which doesn't make much sense on it's own. You could indeed make it so it's only accessible once you go to the buildings resource, so
GET someapi.com/buildings/buildingTypes could be the way to navigate to it.