Repairing rotten joist ends

[Doggit]

From you're comment regarding the state of the skirting, I ssume there's a gap under it. If so, then Lowers comment about jacking the joist up to level them is a good one. But you have to be very carful how you do it. It'll depend on how much height between the earth, and the joists, but you could use concrete blocks to give you a stable base, and then use a car scissor jack resting on a piece of wood on the top to get things level before tightening up the bolts. Vinns right when he says that dog tooth washers should be used between the joists, but I always found that they pushed the joists apart, and I preffered the fricton of having the joists tight together.

 

[vinn]

OP,
There are a few bits of missing information that would help us to help you.
Why not post pics of what you have now - and then expose the joist tails & post pics of what you find?

Are the joist pockets located in an outside wall?
How do the concrete block partitions fit into the scheme of things?
Is the present floor surface level two ways, & flat?
Is there cross ventilation present below the floor?

Can you go crawl under the floor & examine the situation not just below the office but under any adjacent suspended floors also?