Using the new website, https://tm30.immigration.go.th/tm30/#/login, (Started mid September and superceded the previous online and Section 38 app) is VERY EASY compared to all previous methods, WORKS WELL compared to previous methods, and requires less information. Much easier to add foreign vistors or tenants too (no need for TM6 info, or arrival dates, and entry point - which, of course, the Immigration always had!.
I had to re-enter the information (which I had previously entered for the old website and Section 38 app) from scratch, but that turned out to be easy. I have a blue house registration so I entered info as a Thai using my pink non-Thai ID card.
All I had to upload was the scan of my Pink card, the copy of the front page of each house registration, and I also uploaded a scan of the Land Deed. Non Permanent Residents would probably upload their passport detail page and a scan of the Yellow Book and Land Deed.
Register your condo/house first, confirm your email address, and start adding any foreign tenants or visitors.
Only problem (same as for pervious systems) is printing a hard copy of the TM30... I just did a screen shot of the acceptance page to print out for the tenant which may be needed for visa extensions.
If tenants/vistors leave the country there seems to be no need to do a new TM30 on re-entry (but I havent had to do that yet) as there is no arrival date, entry point, or TM6 number to enter.
Don't waste your time going to an Immigration Office etc.