I would not do it.
These used to be more common but they turned out to be death traps. The fumes from numerouse solvents (gas, brake clean, carb cleaner) collect in the pit and eventualy overwhelm the person in the pit who then collapses in the pit and sinks to the bottom where there is even less air than there was at the top. The other thing that can happen is the volitile solvens explode in the pit.
Based on the above insurance companies stopped insuring places with pits, and hence the decline. This is probably why somone suggested checking with your insurance company before going to the trouble of building one.
In the end I decided to get a lift becuase of the above issue with the pit, but also becuase it came out cheaper than building the pit. Now if you do the digging perhaps a lift would not be cheaper, but when I looked at all the digging I said screw it.
Leo