The diagram shows the basic requirements to install a door set. Ideally you should put in a sub-frame made of 30mm softwood or 18mm ply, screwed securely to the brickwork. In a new build this enables you to make an accurate opening and can plaster up to it. Then you will be able to install these door sets at the last minute, after all decoration has been completed.
As the diagram shows you should allow 90mm on the width and 50mm on the height (from the finished flooring height). So, if the door is 762mm wide, the opening is 762+90mm =852mm. With this sub-frame in place it allows you to fix the veneered frame in ideally with corrugated staples using a high powered staple gun. Our preferred sub frame material is 18mm plywood, which should be full width of the finished wall thickness. These will be hidden once the architrave is on. All hinges need to have at least one screw replaced with a longer one to secure the door set into the sub-frame/ opening. You can also put in longer screws through the striking plate. This method of fixing will leave the frame without any noticeable fixing marks.