Repair or Replace shed floor

[RichieS64]

I have a couple of problems here so any help will be appreciated, My garage is roughly 5m x 5m and the floor is r/s, it has sunk on one side by 50mm plus and on the opposite side it has a crown in it as well as sunk at the front edge, there are also several large cracks running through it approx 40mm wide. Also the neighbours behind us have decided to build their garden right up to the rear wall and everytime it rains I get water running through the shed as well as coming up from under the slab. Trying to approach the neighbours about this is a waste of time as they are (I wont go there as I will be thrown of this forum).
My questions are,Could I cut one side of the shed floor out and repour a 1/2 slab and some how tie it into the bottom plate of the shed wall and then do the same to the other side at a later date???

How and what would I need to drain the water from under the slab??/

What would be a rough cost of this and is it worth attempting??

Thanks for any advice it will be greatly appreciated,
Cheers,
Richie

[Dustless]

I am no drainage expert, but I used to repair concrete professionally so I saw a lot of problems like yours. The water in the soil under the slab is what caused it to drop and if you do not fix the soil moisture it will happen again. So it is critical that there is no free water seeping under the slab.