I’m fairly sure that all newly built houses in the UK require solar by law.
It is very poorly implemented. “Builder grade” solar panels in a “smallest compliant” configuration with no concern for architecture to benefit from solar takes place. Builders are intentionally putting the shittiest solar to reduce value of the homes they build so that they can complain about the policy.
All the new houses around here with no solar would indicate that is not true. They’re not even required to have a south facing roof.
At least here in California, having solar panels on a non south facing roof usually only reduces production by 10-20%, as long as it’s not entirely north facing. Solar systems are often slightly undersized - it’s more cost effective to size it so it handles average load rather than the summer peaks you only see for a few weeks per year - so the actual difference for a given system may be less.
With my system, I see the best output from south-east facing panels since they get the morning sun. West facing panels are also fairly popular here due to time-of-use electricity plans. Some electricity plans have peak pricing from 4 to 9 pm, so people want to try and collect as much sunlight as possible during that period before sunset.