What makes you think your architect can't draw on the blueprints?
The designs are available on GitHub:
https://github.com/wikihouseproject/Skylark
They're marked as being made available under the terms of the Creative Commons Attribution-ShareAlike 4.0 International licence[1]. The page also says you have to agree to the 'EULA'[2] of WikiHouse, but these explicitly give precedence to the CC licences:
All content should be considered to be protected by copyright, with all rights reserved other than those materials that explicitly carry another licence, for example an Open Source or Creative Commons licence. In these cases, that licence shall be clearly marked on or by the relevant media, file or repository.
If you enter into a private agreement with your architect to build upon the WikiHouse designs, the most usual case is that you hold the copyright over the changes to the blueprints which you commissioned. Due to the terms of the CC licence that you agreed to, you must licence your modifications under a compatible licence
if you distribute them further. Likewise, if your agreement with the architect gives them title to the copyright, this responsibility is initially on them.
I don't see how this arrangement could be considered objectionable?
[1]: https://creativecommons.org/licenses/by-sa/4.0/legalcode.en
[2]: https://www.wikihouse.cc/terms