To make this clear, I am also very fine with staying were we are. But why not have a look around.
Below are the contacts which my colleague Hella Hernberg provided. In case you write an e-mail to anyone, feel free to refer to her and me, that might make conversation easier.

Also it might be good if we either out others in CC or make a note here once we have called a place, otherwise we will do everything double or triple.

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Hella's email:

The Lapinlahti hospital might also be an interesting location, if they have available spaces. https://tilajakamo.fi

City of Helsinki / Forum Virium has been running a project about flexible spaces, mostly in Kalasatama. This project is about more short-term use (hours-days) but the people might be aware of some suitable spaces and they have connections to City of Helsinki real estate department. Maija Bergström is a contact: maija.bergstrom@forumvirium.fi 0407661415

In connection to this, there are companies/start-ups specialised to operating this kind of flexible use, such as Kliffa: http://kliffainnovations.com/whatsup/pasilanalku/ They just recently opened this shared office in East Pasila, ground floor of a residential building owned by City of Helsinki. These people also might have some ideas of spaces.
Try for example charlotta@kliffainnovations.com

The Museum of Finnish Architecture owns a wooden villa in Kaivopuisto, which has been empty for quite some time as far as I know, but occasionally there are events. Maybe there could be a space to rent. The museum director is Juulia Kauste: juulia.kauste@mfa.fi 04577311233