New slab up against wood of old house a problem?

Yes. Wood in contact with concrete should be pressure treated wood or it will eventually rot. If it is not pressure treated wood there should be a flashed/waterproofed barrier. At a minimum, assuming the crawlspace is dry, some 30# building felt, roll roofing or roofing membrane should have been used in your situation.

I don't understand why the two slabs don't align. Perhaps there were reasons other than not wishing to excavate additional material.

ok lets see if we have this right

you have a house that has a crawl space

the contractor built an addition on to your house and did not match the floor levels of the crawl spaces

additionally he The new slab is up against the new block on 3 walls, but on the 4th wall the gravel and concrete slab are up against the wood of the old house.

sounds like you got a a bright one there

so you are saying where the addition meets the house they poured concrete and gravel against your wall and didnt even weather proof wow

well it is too late now to do anything really I hope they at least left the siding on the house where they poured concrete and you arent saying they poured it against OSB

About the only think i can think of is go into the crawl space with a few cans of undercoating that is used on cars and spray the crap out of that wood If not automotive under coating then try to find another good product

they should have at least installed some rubberized water and ice barrier but really they should have matched the old sill level with the new sill footer and used some rebar to tie them together.

anyway good luck and if it is a problem it wont happen for a couple years