Built or Building?

Question: One of my students had an excellent question. He wanted to know why we call a building a ‘building’ when it is finished being built. Given the use of -ing to indicate continuing action and verbal nouns, it seems a bit odd that never came up with a different word for a ‘building’ once it is completed.
 I was wondering if you have any insights.  (Teacher English Language)
 
Kate's response: Great question! The original job description of -ing was to form nouns — at first nouns of action but then during the medieval period the suffix expanded to include a broader range of nouns, and also included things which resulted from the action (like building etc.).

It’s interesting that the earlier form in Old English was -ung and the suffix that formed the past participle of veerbs was -ende — somehow these two endings collided to form the -ing we have in the modern language (so two suffixes merged and hence we find today a rather confusing array of functions for -ing).

Hope this makes sense!