The 20th international Passive House conference will celebrate the 25th anniversary of Dr. Feist, Professor Adamson, and Dr. Ebel's first experimental Passive House.
This international Passive House conference will likely be the lowest-cost option for many years since it costs less than $10 to travel from Frankfurt to Darmstadt, and Frankfurt is Germany's busiest international airport (and the fourth busiest in Europe).
And perhaps one of the best values since Darmstadt is a low-cost, convenient 1.25-hour train ride from Heidelberg where you can visit the city district of the future, Bahnstadt. Bahnstadt is an entire city district which is 100% Passive House construction for 5,000 residents---and the commercial, institutional, and government buildings serving them. Also, it is an easy walk from Bahnstadt to the old city where you can tour one of Europe's oldest university's and most famous castles.
If you want free advice on traveling to/from Frankfurt and touring Frankfurt, Mainz, Darmstadt, and Heidelberg, do not hesitate to contact me. tad[dot]everhart[at]comcast[dot]net