In either the main hall or the enchanter's tower (if I build one), the option is always greyed out. Specifically, it shows the soul gem V requirement greyed out, even when I have several in my inventory. Again, I do have all of the required materials in inventory, but for this use only, the soul gem V doesn't register.

I haven't read about anyone else having this problem. I guess maybe something to do with one of the inventory mods I use. But lots of people use them, and no one else seems to have the trouble. And for that matter, this is the ONLY time I run into this sort of issue with anything in my inventory.

Anyone have a fix or a workaround?

There is no Hearthstone mansion in Skyrim. If it's a mod, this should be in the mods section of the forums and you should ask the modder.

Thanks, Rosveen. I thought it might be something dumb like that, but I couldn't figure it out.

Thank UESP. I've never even built a house.