Is it possible to change the thermostat without taking the whole front end off? i.e is there enough room.

On a D3 2004 A8

 

I've done an entire timing belt job (including therm) on an '01 D2 with the front end in place (others have too).
Plenty of room for everything, BUT haven't read about anyone doing a D3 yet (with or without front end removal)...different body shape up front, so don't know(?).

 

I'd be surprised if it was possible.

I've worked on a lot of vehicles over the years, but I can barely fit my forearm between the front end and the serpentine on my D3.

 

Well the serpentine is the first thing to come out, with the fans (both electrics in D3?) ...that's the easy part (relatively) and frees up huge amounts of space for the timing belt. Bentley says the front end has to come off for that with D2's and it's a piece of cake with the front end in place..
I've even done a B5 2.8 belt with front end in place..much, much tighter than D2, but doable....so I wouldn't rule it out completely.
Don't really know without looking at one in person, though...but DO know, you can't believe everything Audi or Bentley tells you, until you've tried it yourself.