aedificium

Aedificium, aedificii, n., second declension: aedes plus the root of facio, so literally a thing built as a dwelling. The verb aedifico is formed the same way. Classical writers use it for structures without grandeur, ordinary buildings; where a temple or a great house is meant they reach instead for aedes or templum, and a medieval charter listing aedificia is listing outbuildings and barns as readily as halls.

The declension trap is the plural. Aedificia is neuter plural, not a feminine singular in -a, so the verb goes plural with it and adjectives take neuter plural endings: aedificia antiqua sunt. Learners who have just met first declension nouns get this wrong more often than any other single form in the second declension neuter.