Took the finished van to a weighbridge and I am over the limit: what actually weighs what, and what can be removed?

On uprating: it is a real option, it is not always available, and it does not do what many people assume.

The general idea is that some vehicles can be re-plated to a higher maximum weight, either because the chassis is already rated higher than it was sold at, or with suspension changes. Whether it is possible depends entirely on the specific vehicle and on the rules where you are, so the first step is a conversation with a specialist about your exact van rather than a general answer from anybody, including this thread.

The things worth understanding before you go down that road:

It may change what licence you need to drive it, and that is the point at which this stops being an engineering question. Depending on where you are and when you passed your test, going above certain weights can put the vehicle outside what your licence covers.

It may affect insurance, tolls, speed limits and where you are allowed to park or travel. These vary by country and they are not always obvious in advance.

It does not make the van drive better. You are raising a paper limit, and the brakes, tyres and suspension still have to cope.

So: get the real numbers first with the split weighing above. A lot of people who think they need plating find they need to remove eighty kilos, and that is a weekend rather than a project.

It went in a hundred small places, which is exactly why you cannot point at one thing. Rough order of what actually weighs the most in a conversion:

Water. A litre is a kilo. A full tank is often the single heaviest item in the van and it is invisible in the build because it was empty while you worked. This is almost always where the surprise is.

Batteries, particularly if they are lead acid. Lithium is a large saving here and it is the one upgrade that pays for itself in payload.

Timber framing and ply lining. Individually light, collectively enormous. Battens, 12mm ply on walls and ceiling, and the furniture carcasses add up faster than anything else, and this is where most people are heavier than they think.

Furniture and worktops. A solid wood or stone worktop can be tens of kilos on its own.

Insulation is usually light. Not your problem.

Everything you put in it afterwards. Tools, clothes, food, gear, books, a bike. People weigh the build and forget the contents, and the contents are often a couple of hundred kilos.

The method from here: weigh it again empty of water and personal kit. That splits the problem into build weight and load weight, and they have completely different fixes.

The changes that actually returned meaningful weight for me, in order of what they gave back per unit of effort:

Smaller water tank. I went from a large tank to about half and gained the most of any single change. In practice I was never running it dry between fills anyway, I was carrying a reserve I never used, everywhere, all the time. Work out what you actually consume between fills over a week rather than what feels safe.

Thinner ply and less framing. 12mm ply on walls and ceiling can very often be 6mm or 9mm with sensible battening. Retro-fitting this is a rebuild, so it is a lesson for the next one rather than a fix for this one.

Leaving the second water tank empty unless heading somewhere remote. Free, immediate, and reversible.

Being honest about the gear. I was carrying a full tool kit, a spare of several things, and books. Two boxes came out and stayed out.

Lithium instead of lead if you have not already.

The habit worth building: weigh at each stage of the next build, not at the end. A bathroom scale under each item before it goes in, written on a list. You cannot manage this at the end; the whole problem is that it accumulates invisibly.