Proper vanity dimensions to match an ordered countertop

[Jeremy Patrick]

Planning for the next project which is a bathroom renovation. My aim is to make a vanity base for the bathroom, but I've run into a knowledge gap on pairing a custom vanity base with an ordered top.

The style of counter we've found is at home depot. Planned out a concept of it a few months ago, but don't have the exact numbers in front of me. Let's say 30x70" for the minute.
What I'm not able to find is if there are specific measurement adjustments for a base to properly accept a counter top. When I was first looking in the stores, I recall one of the cabinet guys saying there was a difference in measurements for what the base would need to be in order to fit the countertop, but it was too vague on what exactly. Didn't try and dig into it too much since I was still a couple months out.
So I'm re-visiting the topic now.
For most countertop undersides, are there specific grooves or meeting points where a base would be secured? Or is the undersurface generally flat and the top of the frame simply sits flush to it pending applying caulk?
I've also seen some articles say to order the top based on the base measurements.

What would you advise is the best way to approach this design?

[Dan Friedrichs]

Aside from any sink protrusions, the bottom of the countertop will be flat and "attaches" to the base cabinet with a bit of caulk. You just need to decide on the outside dimensions of the cabinet (and since it's a stock countertop, it probably fits a stock cabinet dimension).

[George Bokros]

The first question you need to answer is will the vanity counter be against a wall on one end, that will determine how long the cabinet needs to be. The typical counter over hangs the cabinet 1 1/2". This all being said then if the counter top will be against the wall on one end the cabinet needs to be 68 1/2"" long, if it is not against the wall on one end then it needs to be 1 1/2" shorter or 67" long. Depth would be 28 1/2" in all cases.

This is how I would approach this situation. Using my info is at your own risk, no guarantees or warranties apply.

[Jim Becker]

I personally provide a 1" overhang of a top on all sides not contacting the wall or other structure. So in that example, the cabinet would be 1" "less deep" front to back and 1" or 2" narrower side to side, depending on whether it's free standing or butts up against something on one side. Etc. That's just an example. The same math applies regardless of what your reveal/overhang for the top is.

[Jeff Heath]

Vanities-

34" tall-or custom fit to your needs. 34" typical
21" deep-distance from wall to front edge of cabinetry-custom fit to your needs
1" overhang on 3 sides is pretty standard and typical around here, but you can choose whatever you like as long as the counter you choose works with the cabinet you have/build.

Kitchen base cabinets

34" tall (to bottom of countertop)
24" wall to front face of cabinet
1" to 1 1/2" overhang typical, depending on door type- flush inset vs. full overlay, etc... No woman or man I know likes the water from the sink dripping on the doors below

You can customize to whatever you like from the standards, but if you're buying a box store countertop, you will need it's measurements, and adjust your overhang(s) from there.

31 years and counting as a cabinetmaker. None of this is set in stone, but these numbers are what is typical in my line of work in this area (upper midwest).

[Jeremy Patrick]

I think I see where all of you are aiming at.

This unit would be up against 2 walls, left and front.
The leftmost area would also have a bit excess, maybe 1" or so to allow for door swing and possible wall variations right?

Then the front and right side would have that 1" or 1 1/2" less depth than the counter?
So 68 1/2 but then subtract another 1" or so for the right side excess, so 67 1/2?

Am I seeing this correct?

[Wayne Lomman]

Jeff pretty much covers it. I would add that you really need the vanity top in front of you so that you know the cabinet fits. Don't assume anything. You also need to be aware of the style trend the top is following. The last few years have seen a trend to flush finishes ie no overhangs. I don't like it or do it but be aware that designers may expect this kind of approach. Cheers

[Lee Schierer]

I've built two vanities that were to receive counter tops with pre-mounted sinks. I had to modify the vanities on site to allow the sink overflow to fit past the top frame of the cabinet. If you can, I would suggest waiting until you have the counter top in hand so you can see how large the under hanging bowl is so you can avoid putting structure or panels where they will interfere.

[Jeremy Patrick]

Good points. Can't remember where I heard it but I think it went, "Assumptions are the mother of all ,er, screwups!"

I was hoping to do more with the base prior to the top arriving as my shop is so small I wouldn't have much space to keep it safe.

On the other hand I do see the point of having it there in hand to see the exact points where a traditional build would have to be adjusted.

Thanks again for the good pointers

[David Utterback]

When I did mine, I bought the vanity top first. Some have a built-in back splash in which case you may need the complementary one for the side wall. Also, will the top be a single or double sink? Just some additional thoughts to consider. Good luck.

[Jeremy Patrick]

The one I have in mind is a St. Paul custom unit from HD. I have it designed to have a builtin back splash on the back and left side. Maybe the left side is an add-on, can't quite remember how it was listed.
The top will have a single sink. The nice thing on the St. Paul builder is you had use a slider to put/move the sink wherever you want. I have it about 3/4 from the left side which would put it about where the old sink and plumbing are.