Probably not as much as you think, is the short answer.
Firstly ignore all the figures published by the manufacturer except for the all-important GVW or GVM (Gross Vehicle Weight or Gross Vehicle Mass) and the axle weights.
The only way of knowing what weight your van can carry is to fuel it up, top up the oil and the screenwash bottle, load it up with the stuff you normally carry with you – straps, ropes, blankets, sack truck, tools, maps, laptop, sandwiches, etc and drive to the nearest public weighbridge and have the van weighed with you in it. You can find your nearest public weighbridge listed on your local Trading Standards office’s website.Subtract that weight (the unladen weight) from the GVW that appears on the van’s Plate (or the Revenue Weight from the V5 registration document) and you’ll have the maximum weight that you can carry.
You could be in for quite a shock: it’s not unheard of for a 3500kg GVW van to have a payload of around 1200kg, rather than the 1500kg that the owner had assumed.
Now have a think about the axle weights. If your load isn’t distributed properly then you could be under the maximum GVW for your van but still be prosecuted for being overweight on one of your axles. This is a particular problem for small vans.
Take the Citroen Berlingo LX for example – the manufacturer’s figures give a kerb weight of 1130kg, a GVW of 1930kg, front axle 1000kg and rear axle 1080kg. With a margin of only 150kg to allow for the incorrect positioning of your (theoretical) 800kg load it would be very easy to be overweight on the rear axle while still being under your GVW.