Tech Support Answer.
Thank you for your email and interest in Home Designer Software. I spoke with my Technical Support team regarding your question below. High Sierra was released after Home Designer 2018 so we were unable to test on the newer platform before the 2018 release. Customers have downloaded the software on High Sierra and our Technical Support team has not seen any pressing issues with performance. However, High Sierra is not officially supported with Home Designer 2018. When our next Home Designer product release is available, it will be tested and supported with High Sierra. I hope this information helps.
I have since done the upgrade and there are no apparent problems.
Personally I think it would help users if the company released a statement when a new OS is released saying either don't upgrade or upgrade should be fine. I also think that a software patch should be issued to make it compatible if need be otherwise the useful time of the program is less than a year.