As @SteamyTea said, we self designed to PH standards, but we didn't have PH certification. I modelled the design using the PHPP and ran many iterations to come up with a design that didn't need a conventional heating system. I found the PHPP relatively straightforward to use, but that was helped by my background in mathematical modelling. This was all many years ago using the 2007 version then updated to 2010 version. I was then a member of the AECB and during get togethers, found some of the Architects knew less than they should, although purporting to be PH experts.