Have a carpenter remove the ill-fitting skirt board and make a new one.

OR

Patch the gaps using something like Durham’s Rock Hard Water Putty; I’ve had great success with that stuff!

Are there vertical as well as horizontal gaps? A 1/2-3/4 inch bit of 1/4 inch thick flat wood trim, prepainted in white, could be tacked on every horizontal edge. Try it on one to see if it looks good or bad (use a cut down piece of a wood paint stirrer stick). Or use prefinished oak that matches the treads.

Glue pieces of wood in the gaps. Then put a coat of Bondo on the outside to smooth it, sand, and paint. Mask off those lovely stair treads. Lowes sells square dowels in assorted sizes, would only require cutting to length. In my 120 yr old house there were some places where there were gaps between the bottom of the baseboard and the floors and that's what I did

That’s not due to any settling. That’s due to some hack hacking at it. Hire a better carpenter to deal with it.

To reply to the comments above:

- Yes, there are vertical and horizontal gaps that are both rather large!

- I didn't pull off any carpeting. It's possible a previous owner did.

Scribe the skirt, pull it, cut it, and reinstall it.

With both horizontal and vertical gaps, you need to fix the skirt board.

It’s all about the (bad) carpentry. It needs new trim work. Nothing to do with settling. Looks to me as if new treads and risers were installed and they aren’t as thick as the old ones.

Ditto what mainenell says. Older treads were removed and new ones installed. Maybe 1/4 rounds/trim, either painted white or stained to matched treads. It will be a time consuming job to get that to work however.

If, you're in a pinch with time and money. Do what Rose suggested. Fill with a block, Bondo, sand and paint. Otherwise, do as everyone else suggested. Replace the skirt boards.

Or do as Joseph suggested. Cut a new skirt board. There are lots of how-to's on the web. Not especially difficult & can be done with basic tools.

This is not settling. This is bad carpentry. The gaps are PRECISELY the same on each and every tread. No house 'settles' that precisely. And if it did, you have more problems with the house than the badly cut stringer.

It's the stringer not the house. Which is VERY good news.

Thanks everyone! Now, I just need to find some skirt board carpenters in DC (who aren't too pricey).

Michele - did you find someone to do this? im nearby and need the same work done.