It's not always possible to fit standard cabinets into your designs, and I had one that required shrinking some 24" base cabinets to less than 22", and 12" cabinets to 9". There are a number of threads about cabinet modifications, but few with pictures of what to expect or how it turned out. Since I was doing a number of them, I thought I'd capture a few pics along the way.
I must say, it was a tenuous undertaking, but it really wasn't that hard to do.
Project A - cutting 24" base cabinets to 21 5/8" deep.
I started by measuring off 21 5/8" from the FRONT of the cabinets to make sure they were all the same size...and the right size (no math involved).
Draw straight lines (a long straight edge is really useful here) to cut along. Make sure you're cutting the unfinished edge (back) of the cabinet off.
Cut all the panels -- 2 sides and the bottom.
Then you have to make some holes for attaching the top rail. I found it easiest to make a template by holding a piece of paper in the 'good' corner, and rubbing with a pencil to capture the hole outlines.
Then poke holes in the center of circles and use that to mark for drilling the holes. (sorry I didn't record the drill sizes, but I matched 'em up by eye anyways -- they should fit IN the old holes).
Assemble the cabinet as normal, using all the usual steps. For the bottom, you need some support where the wood peg is lacking, and rather than try to match up new holes for the pegs, I just pre-drilled and used a particle-board screw (bigger threads than normal screws to dig into the particle board better). Note - center the screw between the two holes on the bottom that the legs fit in, or you may have a problem later...
There are some discussions about whether 21 1/2" was enough for the drawer dampers, so I went an 1/8" more to 21 5/8" but that wasn't really needed.
Project B - cutting 12" wall cabinet to 9" for use as a base cabinet.
NOTE: since I was using this as a base cabinet, I made NO allowances for wall mounting, so you're on your own if you wanted that.
As above, measure 9" from the FRONT because these panels are different sizes! I was doing three cabinets, so I used a table saw, but I cut the larger cabinets with a circular saw, so either would work.
The challenge with these smaller cabinets is that you are cutting the back set of shelf holes off, so I used the cut-off pieces as a guide, clamped to the back edge of the side panel, and drilled through them to make new holes in the right places. I used a drill press for this, otherwise you will have trouble keeping the drill vertical and stopping at the right depth.
Top and bottom are simple panels, so I used the particle screw method to secure the back corners together (actually, I used some decking screws I had in the basement).
In my case, all the cabinets are covered with panels or against other cabinets on all sides, so I was only really worried about the inside surfaces, and then not too much since all my cabinets have solid doors. The holes are the most obvious area to mess up, so I would strongly recommend a drill press (rent one) and take your time. Get them wrong and the shelves won't sit right (you'll need to make some notches for the shelf pins). I did three cabinets in a couple of hours, so it's not really a big project. If you are renting a table saw, remember to cut your shelves as well, while you have it.
Hope this helps others get over their reluctance to cabinet mods. I saved well over $1100 in counter costs by doing this, and I should have a better kitchen because of it -- and that's what it's all about! Finished product below shows what I got - 12 3/4" of overhang for the seating area (remember the 5/8" coverpanel) and 24+9 back-to-back cabinets all under a 36" countertop (25% cheaper than a 37" countertop based on the suppliers formula).
