Living in Caravan

[Rip]

Municipality has obligation to offer various services to all official residents (some of them are free or the fees at least won't cover the actual costs), and some of them are even directly related to spot where you are living (help to elderly people living at their homes, transport to school for kids), so they want to control where people are living.

If you own a another, "proper" house/apartment a town will not (often) let you get officially domiciled in a cottage ( http://www.hs.fi/kotimaa/a1382163384182 ). If you have no other home at your disposal, then it gets trickier. I am not sure if all the relevant legal aspects have been confirmed by the courts.

Living in a caravan also requires a permit from the municipality

Toimenpideluvanvaraiset toimenpiteet

Edellä 126 §:n mukainen toimenpidelupa tarvitaan sellaisen rakennelman tai laitoksen, jota ei ole pidettävä rakennuksena, pystyttämiseen tai sijoittamiseen taikka rakennuksen ulkoasun tai tilajärjestelyn muuttamiseen seuraavasti:

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3) asuntovaunun tai -laivan tai vastaavan pitäminen paikallaan sellaista käyttöä varten, joka ei liity tavanomaiseen retkeilyyn tai veneilyyn (liikuteltava laite);

https://www.finlex.fi/fi/laki/ajantasa/1999/19990132

[Rip]

But suppose if I buy a plot of land in residential area and plant a caravan instead of building house? Its interesting to hear what other risk consists of.

As linked above, that requires a permit.

[Upphew]

The article you linked says there are still lots of families residing permanently in summer cottage. One risk I could think of is during winter the roads in between the forest to summer cottage not being up-to standard for driving (in case of emergency or ambulance or so)

You live where the computer says. Summer cottage needs building permit to make it home. Without the permit computer won't accept that as your home address. There are rules about response times, health care, schools, daycare etc. So you can stay at your cottage 365/year, but you'll pay higher property tax and can't demand those services.

Usually the last km or more are private roads. They are kept in such shape as the owners see fit and are willing to pay.