I agree if you can't go the full depth what is the issue , you go what the space can handle. I never can understand the huge amount of stuff people store in bathrooms so 3" loss of drawer space would not be an issue for me. Maybe post thre floor plan here if tyou want real help.

Virtually every standard vanity newly custom built, purchased online, or wherever is a cabinet depth of no more than 20 or 21"

What are you trying to stuff in the drawers, which simply need to be full extension?

Sounds like you should post the bathroom plan in total? : )

Hi, Jan is right the standard depth is 21" which I believe is absolutely enough for a vanity. Posting a layout of the bathroom and maybe explaining better why you would like something bigger might help. In my opinion 21" large enough.

Drawer glides come in 3" increments, and fit cabinets 3" larger than their specs. So a 24" deep cabinet uses a 21" drawer glide. A 21" cabinet uses an 18" drawer glide. This is industry wide standard construction detailing. You can shift a little bit to fit a custom depth, but those are the standards, because that's the materials that are available.

Stick with engineering and let the the kitchen and bath designer design them, and the cabinet maker make the cabinets.

So not true. Undermount drawer slides come in 3" increments. Blum undermount slides require about 1" more in depth than the stated runner size, which means a 21" drawer needs an inside depth of about 22". I don't know what the exact specs are for the other brands, but I expect they're probably about the same.

Side mount drawer slides come in 2" increments, and at least for Accuride the slide itself is about 1/4" shorter than the stated size.

So if you have 22" of inside depth, you can have a 21" drawer with undermount slides, or a 22" drawer with side mount slides. If you can't quite get the 22" of required depth, with a side mount you would only have to size down to a 20" slide, not an 18" slide as would be required for the undermount. Another advantage of the side mount is you can actually make the drawer deeper than the slide size. For example, you could have a 21-1/2" drawer with a 20" slide. The last inch and a half of the drawer wouldn't pull out all the way, but it's bonus space.