Presumably named for the super-ecological Passivhaus concept from Germany, it is an ingenious way of building efficient houses in high volume. This is what the finished product looked like: it cost about half a million pounds although that included prototype work.
The idea was that the floors would be built on a grid basis to make things quicker and easier -- rather as in Japan rooms are measured by the number of tatami mats needed to cover the floors. Hence wooden spans were noticeable at set intervals in big rooms. Personally I found it a pleasing pattern.
Alan also applied for a patent, in May 2010 (odd if the episode was shown in 2009, as patent applications are made for new concepts, not those disclosed in TV programmes). An international patent application was published as Pre-fabricated building structure. Here's one of the drawings from it.
It was, however, withdrawn in 2012 as a European patent application and doesn't seem to have been granted elsewhere. This was presumably because three patent documents, two French and one Dutch (but in English), were found to have anticipated what Dawson's application was claiming protection for, as listed in this list, which also includes patent documents he was aware of. It shows how hard it is to look for prior art even though Dawson used a patent attorney. The French documents were more than 20 years old and hence could not be used to take legal action against Dawson, while the more recent Dutch application turned out to also be deemed withdrawn as they had not replied in time -- perhaps the list of documents in the European Patent Office correspondence offers hints. This was the sort of thing I was looking up for people all the time when I worked in the British Library.
Adaptahaus has its own website.