Just to give you a litlle more peace of mind! Cork County Council learnt to their detriment a number of years ago that its human nature/the nature of construction for there to be very small differences between drawings and what's actually built.
I was invloved in a case (on the periphery) where a developer built a scheme of houses and there were some differences between what was built and what was on planning drawings (significantly more than 4" I can tell you - we're talking in terms of feet in both plan and height) for various reasons including topography and the competence of the person who prepared the planning application. CCC pushed enforcement proceedings all the way to the High Court (being pressurised by on particular local resident) but were slapped on the wrist and derided by the judge for bringing such a case. He ruled that the development was generally in keeping with that granted permission and because it caused no significant additional negative impact to the adjoining properties the development as built was basically in complaince with planning permission granted. He also told CCC that a certain degree of tolerance had to be applied when comparing as built structures with planning drawings - common sense should prevail!
Just to add, I am not saying for a minute that people should have carte blanche to get planning permission and then build what they like! But in some cases the as built structure, for legimate reasons can differ slightly from that granted permission. The case above was decided on it's merits.