We had to hire an energy consultant anyway due to KfW40+ / Passive House. Since there are extremely many interfaces between the architect and the energy consultant, we thought it would be a good idea to hire someone who is both. That worked out very well. The construction project, a semi-detached house with a total of 4 apartments, cost a total of about 700K without land and exterior facilities etc. We paid 31K (both together) and each of us was reimbursed 4K (proportionally for the energy consultant). That leaves about 11.5K for each for one half of the semi-detached house. We never calculated what that would have cost according to HOAI, also because thanks to the special manufacturing technique, part of service phase 5 fell into the scope of the shell builder.