Oh ok. So it's not "that" much of an issue then as you'll be rendering on blockwork.
Just out of interest, how much bigger would you like to go?
In the past, I have had contractors build something maybe 50-100mm at the absolute MOST wider than what was noted on the drawing, which I'm obviously not happy about but it's the risk they take. Worst case, planning could stop work on site, serve an enforcement notice and ask for the building to be removed and re-built as per the planning approved drawings.
In some situations when actual dimensions are not noted on the planning drawing, then scaling from a 1:100 drawing, there may be "some" tolerance to go a little bigger, but again... it's not recommended because that is not what planning permission has been sought for.
If you're only talking about 50mm, then I would say that's fine as it'll be very difficult to notice that externally... from a planning officer's point of view. And in some cases, speak to the planning department to ask them that you need to go 50mm wider because of some VERY GOOD reason and they may agree and say proceed. But then you do stand the chance of bringing it to their attention, them refusing the reason and then they may carry out physical dimensional checks to make sure the building is the correct size.
