What usually happens under a staircase? Not much. It's Either use as storage or completely ignored. Are you looking for a place to work at home? The space under the stairs ca be the perfect solution
Are you struggling to find a space home for your office? Reality is that during Covid working from home is a must for many of us. Also, once the coronavirus crisis passes, it is likely that many will continue to work from home. Change is here to stay.
A space without isolation
In the simplest version, you can put a normal desk under the stairs. If the space is too small, a table and some light are enough. More than a separate space, it's an integrated into your house.
To make it fit extremely harmoniously, it is worthwhile to think about the color-matching with the resto of your space.
A space with insulation
If you prefer the maximum possible insulation (and the space under the stairs allows it), you can build a mini-cabinet equipped with a door.
If the space is at least 5ft (or 1.5 mts) wide, although small, it is possible to achieve sound insulation.
Perhaps it should not be completely separate, so as not to feel enclosed, but it can be done with thin wooden or metal shelves that allow you to arrange books or small items and that visually isolate you.