I am currently updating my 1960’s bungalow that has had two previous extensions. These extensions now block the few air bricks that were at the back of the house, leaving only one large brick at the side of the rear elevation. The front elevation only has three small air bricks over a 10m span.
The ventilation could only be considered below standard, but I question whether or not they are needed at all if I am about insulate and seal the underfloor. The crawl space is adequate for working in and is accessed through the utility room at the back of one of the extensions.