If you look at the Old English (byldan) and Middle English (bilden) words for building, you will notice that they do not include the -ing suffix. This word eventually morphed into byldinge (now with the -inge suffice). This suggests that the word may have been a mispronunciation that eventually came into common usage.
Another way to look at it:
-ing\ 1. [For OE. -and, -end, -ind, AS. -ende; akin to Goth. -and-, L. -ant-, -ent-, Gr. ?.] A suffix used to from present participles; as, singing, playing.
2. [OE. -ing, AS. -ing, -ung.] A suffix used to form nouns from verbs, and signifying the act of; the result of the act; as, riding, dying, feeling. It has also a secondary collective force; as, shipping, clothing. <===
Note: The Old English ending of the present participle and verbal noun became confused, both becoming -ing.
3. [AS. -ing.] A suffix formerly used to form diminutives; as, lording, farthing.
So it could simply just be a noun formed from a verb (like feeling or gardening).