What a charming house! I've not seen your bath situation before, and can't help with cabinet makers. The quotes do seem extraordinarily high to me, though. One option, as you decide what to do, is to use fitted skirting around the pedestal sink to hide the quirky flooring. (If you design a skirting that you like, you might even want to keep it, and can build a small shelf underneath for storage.) Some examples and one "how-to" link:

Handsome San Francisco · More Info

Sink Skirt with Box Pleats · More Info

https://www.onekingslane.com/live-love-home/skirted-sink/

First, 5500 seems extremely high, and my guess is someone didn't want to be bothered. The lower, depending what you were asking for doesn't seem horrible. Some additional photos of the bath would be helpful and is the vanity itself the only spot for storage.? The "box" under your pedestal, clearly a roofline issue.... might have the possibility of a reduction with a flitch beam. Either way, it would seem all you are asking is a box, open at the back, with some drawers on the side and doors in the front. And a counter top of course, deep and wide enough to bypass the "offense" in the floor. Have you considered a wall mounted vanity with drawer as a ready made option? Perhaps with a medicine cabinet for additional storage? Could be lovely atop a classic hex or penny tile /white floor and just consider that a charming quirky feature, unique to your home : )

http://www.modernbathroom.com/bathroom-vanities/fairmont-designs-shaker-americana-36-wall-mount-vanity-polar-white__1512-wv3621.aspx?adwords=11&gclid=CMiNsaK4gcwCFUxZhgodiwsGEw

http://www.bathvanityexperts.com/wyndham-axa-wc-r4300-36-sgl-wht-resin-35-inch-glossy-white-modern-single-wall-mount-bathroom-vanity-set.html

[https://www.houzz.com/products/kubebath-tona-wall-mount-bathroom-vanity-ash-gray-36-prvw-vr~41442771[(https://www.houzz.com/products/kubebath-tona-wall-mount-bathroom-vanity-ash-gray-36-prvw-vr~41442771)

Obviously you can't steal a foot or two from another room or sacrifice another bedroom on the 2nd floor for a swanky big main bath?

I would be looking at repurposing a piece of vintage furniture or having a self employed woodworker/carpenter make one for you instead of buying a high priced vanity that may have to be chopped anyway.

Agree with the purist, can't imagine why it is there unless a part of framing issue with the pediment over the front door, since you mention it's between a master and stair landing. Rather than consult with cabinet makers, an onsite visit from a structural engineer may first be in order. You may be able to eliminate the offender.

I'm seriously impressed with the plumbing for that. Quite the feat to get that to drain and be code compliant and hidden all in one.

Can you provide a floorplan? I'm trying to understand the angled wall to the left of the sink area. Also, what is the clearance between the sink and the tub (?) opposite?

Very charming house

The placement of the sink in that location was actually quite an ingenius way of making the best use of that awkward bump out!

Is there any wall space where you could have a built in cupboard placed? It would be quite shallow of course....only four inches if you made it the depth of the wall or a little deeper if you had it protrude into the room a bit....but would still be enough to allow the storage of cleaning and bath products. Towel storage might be a large basket with towels rolled up, in it. Then you could just keep the pedestal sink, which I think is attractive and looks period for the house.

This happens to be a photo of a wall cupboard in a kitchen but the same idea applies for the bathroom. Shorter or skinnier for whatever your needs are...

You could get some storage by installing a cabinet base, especially if you made it 36" high rather than the current sink's 32". You could also make it look slightly less awkward if you made the door a couple of inches narrower (don't know how wide it is now). Also, keep in mind that you'll have to be creative with the toe kick to keep the cabinet from sticking out too far.

Be sure to demo the tile and underlayment and wallboard/plaster/lath underneath. You could gain a very important 1 or 2 inches between the framing members and the door frame.

I'd certainly do the cabinet.

HI -- What about re-moving the slanted DIY section and then just box it out and then a two drawer vanity can be put on top . This can be done with a wall hung vanity . Then the stair bulkhead will be the base of the vanity . Make it the same finish as the vanity and not like the flooring .Then if you add vintage style hardware you can make it fit the vintage style . The way it was done with the slant makes it look like it was a cheap solution . Or find a kitchen and bath designer to rework the room and get a quote first .

I don't think even with a custom box, you will be able to put a side on with the door moulding right up against the slanted thing. Its not going to look good and you'll get so little storage it won't be worth it.

HI - @ suzyq53 -- so what else then ? The slanted part might not need to be as big as it is now and there doesn't seem to be another solution other than moving the sink or moving the bathroom. Nothing is known at this point until the slanted thing is re-moved or uncovered to see what is going on there. As an added note ... that does not look like a proper transition strip on the door threshold either . Looks like a trip hazard .

How did it all turn out? Any photos to share?