Berlin - An initiative pushing a public referendum to expropriate the private landlords of approximately 243,000 Berlin flats are using very different figures than those of the state government, which has neither supported nor rejected the concept. 

The initiative, Deutsche Wohnen & Co. Enteignen! (Expropriate Deutsche Wohnen & Co.!), on Monday said the Berlin government would have to pay the landlords €8 billion for their assets, which would be financed from what it calls "fair basic rents" of €3.70 per sqm.

But the Senat, as the Berlin government is known, said earlier this year that such a move would cost far more than the activists estimate - namely €29 billion including a €6 billion cash subsidy out of the stretched city-state's coffers.

Last year the initiative collected 58,307 signatures on a petition for a public referendum to force landlords with portfolios of 3,000 or more flats to hand the assets, most of which were sold over a decade ago by the city-state, back to Berlin. The government in September said it supported the idea of increasing the amount of publicly owned housing in Berlin but stopped short of throwing its weight behind the repatriation idea.

The proposal is now with Berlin's elected officials. They have four months to decide whether to accept the proposal and move forward with it, negating the need for a public vote, or reject it. The activists then would have to collect signatures from seven per cent of Berlin voters within four months, or about 175,000 autographs, to force a vote.

Referendums have a mixed history in modern Berlin. Voters in 2014 rejected a proposal to build on the edges of the closed Tempelhof airport. Although the referendums aren't legally binding, then-mayor Klaus Wowereit said he would follow the will of the voters. 

Another 2017 referendum aimed at keeping Tegel open failed, however. 

While announcing its calculations Monday, the activist group unveiled an online calculator that is essentially a mortgage calculator to allow people to see how the amount of damages owed to landlords such as Deutsche Wohnen (Berlin's largest private landlord) would affect rents if the properties were brought back into state ownership.

At the level estimated by the Senat, Berlin would have to charge an average €7.90 of basic rent per sqm, at the top end of that allowed by the government's own rent cap.

The initiative is basing the referendum on a passage in the country's constitution that allows eminent domain for the public good. A law is needed to set the value of anything confiscated by the government.