Converting storage to residential use

[Rip]

Direct knowledge thin and no interest right now for excessive googling, but "common sense" says it must require both the approval of the house (what kind of majority?) and the city.

Conversions of attic space to apartments are fairly common and they require city approval that is applied on name of the house, and it seems it is pretty clear it also an issue that would need to be updated to "yhtiöjärjestys".

[Upphew]

Can a building refuse permission to convert a workspace, who's in charge, the building or the city or the laws regarding dwellings?

You don't buy the workspace, you buy shares to (housing) company. To convert the space you'll need building permit and that can only be applied by the company.
For better Google-Fu try "käyttötarkoituksen muutos"

edit. place -> space

[betelgeuse]

Direct knowledge thin and no interest right now for excessive googling, but "common sense" says it must require both the approval of the house (what kind of majority?) and the city.

Two thirds majority.

http://www.finlex.fi/fi/laki/ajantasa/2 ... 1599#L6P34

In some exceptional cases the neighbors might have the right to deny the conversion:

http://www.finlex.fi/fi/laki/ajantasa/2 ... 91599#L5P3

I'm interested in buying a property in an apartment block that's currently being sold as storage / workspace, but the agent writes that it's possible to convert it to residential.

Make sure whenever making offers / sales contracts that they are always conditional on the approvals and the payment schedules reflect that. You might want your own expert to make sure nothing goes wrong.

Would it actually be legal for someone to live in one of these properties temporarily if their putkiremontti is going on? Yes, I realise people could just do it without following the rules, just curious if it's actually lawful.

This might be a similar case to the people registering themselves to the summer cottages. Maistraatti might allow it but the municipality has the right to deny living there year around:

http://www.taloustaito.fi/Arki/Kesamokk ... -kunnassa/