My son purchased a home built about 1920. It has a small "shelf basement." He would like to put in an egress window and a bedroom. How were "shelf basements" constructed, and can he remove part of the shelf? Does the exterior foundation wall go down all the way to the basement floor, or just a few feet? What would be required to replace that wall with a standard cement block wall?

I am not sure what you are talking about.

But sometimes house where built with crawlspaces or very shallow basements that where later dug out to make a full basement.

There are several ways of doing that, but one common way was to build a NEW wall insside the existing foundation wall and that wall does down deeper.

That makes the side smaller (ie, the shelf), but it does not disturb the existing footing under the foudnation.

Or it could be butress walls that where installed to help support a failing foundation.

You really need some one that can look at it. And best be someone that is familar with homes of that age.

Also will probably need some digging to find out exactly what was going on.