De or het building?

The correct Dutch article is:

de building

Rule Words ending in -ing: almost always de. See the rule
Meaning (Dutch) (België) een gebouw met meerdere verdiepingen

How to use building

the buildingde building
this buildingdeze building
that buildingdie building
our buildingonze building
your buildingjouw building
every buildingelke building
which building?welke building?
a big building / the big buildingeen grote building, de grote building

Dutch words ending in building

teambuilding | imagebuilding | buikbuilding | bookbuilding | bodybuilding

In compound words, the last part decides the article. See the rule

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).