I'm thinking of putting a canopy over our back door, and possibly
building a covered walkway between the house and our garage, the walls
of which run parallel and about 3 feet apart. However there's a
problem in so far that the central heating boiler chimney exits
alongside the back door, and this would end up with the chimney
venting into the canopy/walkway. Obviously I'm not going to do that!

There's also an extractor hood vent the far side of the back door, but
I'm more comfortable with extending that if I need to.

Here's a couple of pictures of our back door with the chimney clearly
seen to the left:

http://www.yorkshireplace.co.uk/Door.asp

I know it is possible to install a vertical extension to a CH chimney
because a place down the road has got one, but never having delved
into this sort of thing before I'm not familiar with what can/can't be
done. I'm assuming that if I were to extend the chimney then I have to
take care that the chimney can "breathe" in and out, plus whatever I
mount around the chimney (in terms of canopy/roof) has to be
acceptably fireproof.

The CH boiler is a Potterton PrimaF if that makes any difference.

Advice would be welcome. Does this require a gas fitter to do the job
to ensure it is done right, or is it in the realms of general DIY?

PoP