just in the process of turning what was a utility room about 3m by 3m into a small 2nd lounge. It is part of an extension and has no chimney brest in it. I was thinking of putting a small multi fuel or wood burner in one corner of it against an external wall, can i just vent the flu straight out of the wall??

You’ll need twin wall flue diagonally through the wall and up the outside for about 5m. Budget around £1100-1300 depending on complexity for the flue materials and labour.

I’d allow £100 per metre, and £100 per fitting (i.e. 45deg bend, storm cowls, brackets (pair)) etc.)

So 5m + 2x short bits out the stove and through the wall = c. £600
2x 45deg bends = £200
Wall brackets/standoffs = £150
Storm cowl = £50ish
£200-300 labour.

You’ll also need to sort out a sufficient hearth material based on downward heat transmission.

Loads of info here
http://www.stovesonline.co.uk/fluedesign.html
http://www.stovesonline.co.uk/wood_burning_stoves/Stove-Floor-Plates.html

Also worth referencing this:-

http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partj/approved

It gives you the full low down on building regs for fires, including hearths.

When we installed our wood burnerwe also had to have it installed by a HETAS registered installer (kind of like GasSafe/CORGI but for open fires)

My home insurance (M&S) would only cover my property if the work had been carried out by a HETAS fitter, worth asking your insurance company what the score is, some might not be too worried?

I would also advise fitting a decent monoxide alarm, a HETAS fitter will fit one as part of the build, but they can vary in quality.

Plus a building regs application – part J is the relevant document