It wasn’t a big deal and wasn’t very expensive. The house is about 150 years old and just has a granite foundation. The central supports for the floor were pretty bug-eaten, although thankfully the floor above wasn’t soft. (Dirt crawlspaces are, of course, terrible as far as pests and overall dampness.)
Our local building code allows you to excavate to place your finish floor no lower than the base of the foundation walls. In our case, that created a limit of 7′ clear — which is fine because at least you can stand up!!
First, they punched a hole in the foundation wall. Doing so under a narrow gable end meant very little weight coming down on the wall in that area. First it was a bit of shovel work, then they they were able to get a Bobcat in. They excavated down about 8′, leaving the requisite 45 degree slopes up to the foundation walls. Put down perimeter corrugated drains and gravel, running back to a new airtight sump connected into the sewer. Put down new 1×2′ center footings, and added strong lally columns to hold up the floor. Then a vapor barrier of dimple mat. Then a concrete slab was poured to just above the base of the perimeter foundation wall.
We were a little worried about the high water table. But it never appeared. So that was good.
Next, added an angle iron to the top of the opening, and finished it off, added a precast bulkhead, sealed it up against the house, put a Bilco door on there, and we were done.
Only $11k plus the cost of materials which was about $3k. So that wasn’t too bad and it’s an infinitely nicer setup, that can in fact be used as a real basement (and, thankfully, a bone dry one).
Anyhow I just wanted to post this because some people wrote in some other posts that this type of work had proven costly and/or troublesome. If you can just find an area of your foundation wall to punch through that’s not bearing any significant load, I think you will find it’s a pretty straightforward process. Just make everything utterly bombproof and I really don’t think you’ll regret it. Awfully nice to have that extra space.