It all depends on the wheel width Paul rather than wheel diameter - going up an inch in diameter, if the wheel width (note: not tyre) remains the same, won't affect offset.
Looks like the fronts are 17" x 7", offset 55, while the rears are 17" x 9", not certain on offset (similar, I think).
So e.g. if you get 18" x 7" at the front, you need a similar offset. But if you go for 18" x 8" wheels at the front, you need to increase the offset by 1/2" / 12mm as Nick has said to preserve the visual look from the outside - but in that instance you'll lose an inch of clearance against the inner liner, so might experience scrub at full lock...
Edit: This is a decent calculator to see the effect of an example wheel...
http://www.1010tires.com/Tools/Wheel-Offset-Calculator#
"No man with a good car needs to be justified"
Blue '08 FD2 CTR - big, ITR-sized shoes to fill...
Yellow '96 NSX 3.0 - oh was it worth the wait!
Black '99 ITR - well, I had to have another one, the first was so much fun. Miss this one even more than #1...
Blue '03 S2000 - SOLD, flawed but fun
Blue '04 Focus TDCi Sport - SOLD, very good fun for a diesel!
Black '00 ITR - SOLD, still missed
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