As in the title, what would be the tare on a 18 tonner chassis with an ordinary flat body, looking at DAF CF or Atego.
You could probably squeak under 7 ton with a day cab.
these are 6 wheeler weights, my old Daf 85CF was just under 10t as a flat, ( low foor sleeper cab , probably heavier as it was stretched) and the frogtrotter weighs just under 10.5 tonnes.
you should be looking at 7-8 tonnes unladen
Bit of a how long is a piece of string question 
The target payload for a four wheeler has always been 10tons, as lorries have become more complicated they have raised GVW to still allow the magic 10ton on the back. So a day cab flatbed with a 22’ body will comfortably get under 8tons ULW, with the right chassis and body and you could get under 7tons ULW, something like an LF55 with a 20’ alloy flat for example 
Thanks all, asking on behalf of a customer, I’m trying to get him to do some of his own locals rather than me having to thread an artic down goat tracks and dead ends 