(1) First, think about materials. The bathroom gets moist so I would stay away from MDF and stick with wood that has a washable stained or painted finish. We did walnut stained rift white oak which goes really well with our creamy botticino marble. I posted pics of my bathrooms last week but don't know how to link to them (called pics of my new bathrooms I think)

(2) Second, consider drawers versus doors where you can and even under the sink, if you have to do doors, think about pull outs. It is so much easier to look at what you have stored from above than bending down to peer inside a cabinet.

(3) Third, think about what you are going to store in the various places to make sure that your drawers are deep enough. A six inch drawer space will give you only about 4" or so of height, so you need to make sure that your stuff will fit.

(4) I don't know whether this is single or double or what your measurements are, but for comparison, here is what we did. We had only 88" to work with for our double vanity and I wanted lots of drawers and TWO hampers--one for darks and one for lights (one of my best remodeling decisions. So, we did five banks--a 15" bank on the ends(with a top makeup drawer and hamper underneath each side) and in the middle (a bank of four drawers, with the bottom one at 12" deep). We did two 24" sink cabinets on either side of the middle bank with two pullouts. We also waited for the plumbing to be installed before letting them do the cutout for the plumbing so they would leave me maximum interior space under the sinks.

(5) Last, consider framing versus full overlay. This is mostly an aesthetic decision. I personally like framed better, and was able to do that in my other bath because I had enough room but had to do full overlay for the master bath 88" vanity I just described. It still looks great.

>The bathroom gets moist so I would stay away from MDF and stick with wood

Well now, I have a horrible particle board vanity circa 1979 and I have to say that despite the fact that previous owners had four small boys there's absolutely not a thing wrong with it except innate hideousness, lack of useful storage, and being way too short. It's still structurally sound after all those years in the moist bathroom in steamy FL, and it's in a bath with no A/C duct, too, and an exhaust fan that probably died about 1983 or so.

I just pulled out my folder for the vanities. The original invoice we got was for all three vanities. This included a much larger vanity for the MB than we had him make, because we originally planned to expand the MB by removing a closet in our bedroom. Though we tried to keep track of the changes in costs along the way, it's hard to make sense of it now. If my DH isn't too crazy busy when he comes home tonight, I can see if he remembers what we'd paid. Our guy had a lot of personal issues at the time, so he would drop out of contact (for months) at times. We ended up giving him $, either deducted from the balances due, or extra, just to get the job done.

He did good work, though in addition to his personal issues, there were a few avoidable screw-ups along the way (mis-drilled holes for the handles, wrong size drawer boxes ordered/made, etc.). He'd then disappear while we were waiting for them to be corrected or replaced. In the end it worked out okay, but it was nerve-wracking at the time (esp. when it held up installation and tiling).

Luckily, DH made templates for the sinks for the guy to work with, and took pics of the original Sonia vanity without its sink (thanks to our very helpful salesperson at the showroom), so all the bolts and holes lined up perfectly. We also went over the design details, in detail, so we were able to avoid having larger problems than there were.

My advice to you, is to make sure you know what you want, or put together what you want with your cabinet maker. Go over all the little details, and have it all spelled out on your contract/invoice--if you want full extension, soft close drawer glides, and if you want Blum or another brand, make that known. Want dovetail drawer boxes? Include that, and all other specifics, too. Get stain or paint samples, on the wood species of your choice. We supplied our own handles/knobs (kudos again to our wonderful salesperson for his assistance with finding/choosing those!); decide if you want to supply them or have your cab maker supply them.

Going over all the minute details doesn't guarantee that everything will go perfectly, but it should help avoid at least some of the potential pitfalls along the way.

If I think of more, I'll add more later.

the type of finish can be very important. See if they shoot conversion varnish. It's much better than lacquers for kitchens/bathrooms.

If stain-grade, consider grain. Differences can be jarring, grain continuity can make a piece shine. Try to see examples of the wood you are going to use in both flat-sawn and vertical grain. While flat-sawn can be appropriate, vertical grain is usually more attractive to the eye as well as being more stable with regards to wood movement.

If stain-grade and there are going to be several (three for example) adjacent banks of drawers, I'll use a long piece of wood and cut the three top drawer fronts out of a single board, the three middle drawer fronts out of another board, etc. It gives a bit of grain continuity across the front of the piece.

If it's a face frame, I'll do the same with the stiles and the rails, trying to cut them from a single piece.

If the boards used for the drawer fronts are wide enough, I'll sometimes rip the horizontal rail(s) off that board prior to cross-cutting it to length for the drawer fronts.

The better lumberyards may stock boards cut from a single tree trunk. Meaning you can get multiple boards or planks cut from the same tree, they're usually stacked or numbered in the order they were cut. It makes grain considerations, as well as differences in coloration when staining/finishing easier to account for.

Then there is veneer. Some shy away from it. But it's appropriate when large pieces of solid wood might compromise the cabinet; carcass sides, or large door panels for example. Or when large pieces of species lumber are simply cast prohibitive.

The best thing you can do to relate your desires to your cabinetmaker is to show him photos. Cull photos from magazines, woodworking sites, furniture store catalogs, etc. This detail is what I want, this is what I don't want.

When building a stain-grade piece, I'll carry a couple of extra boards through the construction process as I build the cabinet. They get the same planing/sanding/scraping as the boards used in the piece. I use them as pieces to test out the proposed finish. Stain/seal a test piece then show it to the customer. Continue if they like, continue on. If they don't, tweak the formulation.

Have the cabinetmaker come to your house for the measurements. Everything in the room needs to be laid out it it's not already installed; electrical, plumbing, window/door trim, toilet location, etc.

Let him do the measuring. You get the measured drawing and make sure the finished piece matches the drawings. You don't want a light switch to end up obscured by the cabinet, or a plumbing stub out to fall right behind a vertical divider in the carcass.