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I have a Garage detached from my house and a long drive that runs
infront of it along side the side of my house. I have been considering
wether or not i could extend the garage by building another garage of
the same size infront of it of equal size.

The outer wall of the extension would be running parallel with my
nighbours fence however as it is a single story extension and does not
breach the boundary it would not change the view dramatically or block
light for my neighbour, so i wouldnt anticipate any objections from
her.

My question is when building an extension does it have to be a certain
distance away from the boundary line?

It doesn't have to be and if your neighbour doesn't object why should
the planners?

What might be an issue is if the new garage is closer to the road than
the house and in front of the building line.

Building close to a boundary raises certain building regulations issues
but that is another matter.

--
Roger Chapman