Putting aside semantic considerations and other possible meanings of the phrase, it is occasionally, in some jurisdictions, used in the way cosmasad implies, i.e. that the specified dimensions are to be strictly adhered to without any deviation (eg. by the application of tolerances, typical ranges etc).
Also, It may indeed be the case as skol suggests that my suggestion doesn't comprise typical architectural jargon, but I only suggested it based on the small amount of information I had to work with and the fact that the collocations επιτρέπεται/δεν επιτρέπεται* η απόκλιση από τις διαστάσεις isn't all that uncommon in the construction industry and other, various fields of design and technical drawing.
*δεν επιτρέπεται seems to be more common (and perhaps more apt) than απαγορεύεται, in these situations.