I have inherited a cottage problem.

The posts on which it stands are starting to rot.

They are railway ties set into concrete tubes - badly. The tube form was
only a bit larger than the tie, and over time all the concrete has split
off the sides, the posts are now rotting at the bottom.

My idea of how to fix it is to dig beside the existing posts one by one
and construct another concrete footing beside it to support a screw jack;
when that has hardened use the jack until it supports the house, and cut
out the rotting post.

I'll need to do this a dozen times but I figure if I do them one at a time
I can avoid a lot of cribbing, a lot of getting under, and risk of
anything surprising happening. 10 of the 12 are on the outside edges, and
it may be that I can leave the inside ones alone.

Can I leave the cottage on the jacks? This thing is about 20x30 feet, two-
storey on one side (enclosed loft), built with local raw-dimension lumber;
I have no idea how big a weight-rating jack is needed. Leaving it on the
jacks saves me figuring out how to replace the jack with something else
while keeping the cottage from moving.

Jacks cost money, and most of them are much longer than I'll need. Does
it make sense to get the welder I know to make me a dozen? The bottom
could be a plate with 4 stubs of rebar sticking down in the corners and a
pipe centered on the top side; the top piece a plate with four corner
holes (for big sucker screws) and a stub of threaded rod going down. (I
saw this kind of thing on the interweb.) Could these be made to work and
left in place? How big for the pipe/rod+nut and plates?