Foul Smell Under the Stairs

ka7e

Old paint can become contaminated with bacteria and it smells really bad - often like vomit/sour cream. A few years ago there was a problem with Valspar paint that people complained reeked of ammonia.

Some recommendations for getting rid of the smell include - washing down the walls with bleach, using a sealant like Zinsser BIN and using charcoal bags or ONA gel to absorb the smell.

SieIso

If it is the paint, would a fresh layer of new paint resolve the issue? 

Eldi_Dos

If it is the paint wash down the walls a couple of times with spray on sugar soap and paper roll. Let it dry for a while then repaint with new batch of paint.
How old was the paint you used?