Ok, thanks for replying quickly. I thought it might be longer to go through those different things.
A few things concerning the settings you posted:
A) Ambient occlusion, in the PPV (not RTAO), has a radius of 15. That’s extremely small, and with such a bright HDRi, I think it is not accurate. It may also be contributing to the noise. Try a minimum of 100, maximum of 250. Increase the Intensity to 0.5 at least.
B) Reduce the Brightness of the HDRi a bit, to no lower than 0.5, but probably around .7-.9. (It’s a REALLY BRIGHT HDRi, lol. It translates to a heavy influence on the lighting of the scene because it has an internal skylight (a skylight always comes with any HDRi and so is connected to its settings in how it affects lighting). I think that’s a part of WHY there’s noise in the first place. Somehow, not all of the lighting info is getting fully processed for the scene, specifically wider, flat surfaces where it is the most noticeable. Though it’s also affecting other surfaces too. Which brings this to another point:
C) Materials on the surfaces - what are the resolutions of the different materials on those surfaces? If they’re 128 or lower, they’re probably too low of a resolution to get a fine-tuned, non-noisy result. But again, it is most likely in conjunction with the other settings for the HDRi and directional light. Which also brings me to another point (HA HA HA, not really funny yet it is if there’s humor to be found in such a mysterious problem)…
D) Even in deleting the HDRi and its internal skylight (if you added a skylight, that means there was an extra, so 2 skylights, and that would probably screw up the lighting / produce noise), the data of the previous lighting build could remain. In effect, it’s not a proper test of what’s causing the noise. One way I’ve read to remove the old data of the last lighting build action is to go to World Lightmass settings, and temporarily put a check in “Force No Precomputed Lighting”. That removes the old data of lighting and switches the scene to calculate only dynamic lighting (all lights would need to be set to movable, and objects too probably). But instead of continuing in ‘force no precomputed’ mode, simply uncheck it after checking it, and it’s back to calculating static / stationary lighting. One other approach to removing the old data is clearing the cache of the Swarm Agent (the application that processes much of the lighting build). So, while UE is opened, there’s a little icon in the taskbar for the Swarm Agent, go to it and look for Clear Cache. Use it, and then restart Unreal Engine. This restarting is after also checking and unchecking “Force No Precomputed Lighting”.
E) Lastly, if the sphere ground plane is far below the house / buildings to be lit, and the skylight is at 158 Z, then 15 would be enough to get the scene outside of the distance threshold. What is the Z location of the scene objects, roughly?
F) The inside of the house looks rather dim, so in decreasing Brightness of the HDRi, offset it by increasing Indirect Lighting Intensity of the skylight and increasing Intensity of the directional light. The stronger the directional light is in Intensity, the stronger the indirect lighting is going to be because every indirect bounce of light is a fraction of the previous cast of light (1st bounce has an intensity that is a percentage of the first, direct cast). But this is based on the materials’ diffuse value. So, a higher base color (diffuse) value in materials results in higher intensity bounces of light. Hence, increasing Intensity of the directional light AND the skylight is going to allow Indirect Intensity and bounces to be stronger, and may even remove some of the noise, while also lighting up those interior areas.
G) Set RTGI (in the PPV) to Brute Force method. Set Max Bounces to 2-4, and Samples to 4-8.
H) Turn SSR (Screen Space Reflections) off for now. It can interfere with RT reflections, which could interfere with lighting on semi-reflective surfaces such as the walls, ceilings, floors, etc.