Is that building regs or Fensa? I know for sure the building inspector we used to have down here wouldn't sign off top hung PVC windows unless they gave the correct clearance of 450mm x 750mm. He was an *rse though and we have a lovely new guy now
You have to bare in mind that they don't (as far as I'm aware) make a 90 degree opening top hung hinge for PVC windows (in the standard friction stay variety anyway), As the window opens the top part comes down reducing the opening between a 1/4 and a 1/2. So technically making the opening worse than butt hinges on a timber window.
