Prevent overheating by keeping the window area to 20% of the floor area. That’s a very old passive solar rule-of-thumb from New England (Eastern USA) where I grew up.
So I was interested to discover that Mark Siddall has researched this same metric for homes in the UK and found it holds true there. He details this in a brief LinkedIn post you can read here.
Mark Siddall is a building science researcher and Passive House expert, well known to most readers of this website. His work on thermal bypass and the issues it causes is epic.
Mark does say the 20% rule is a whole-of-house guide and suggests doing a Daylight Factor calculation to understand the room-by-room results. This can be done using PHRibbon if there’s a PHPP file for the project.
I’ll check this sometime against a dynamic model. At Sustainable Engineering, we usually just look at the glass-to-floor area for the room. If it is much higher than the rest of the house we tend to add external shading or specify low g-value glazing.