Modernist ideas were written into the building code of 1931 and increased over timewhere traditional cities were banned and supposed to be turned down. How managedmodernists this in the critical juncture. One of their proponents was StockholmsByggnadsförening, where builders, contractors and private and public plannersand architects meet. My questions are how modernist architects could imposethe views of Byggnadsföreningen and if their activities in this organization couldexplain why modernist ideas so fast could impose building code and legislation.I found that it was a self-amplifying process where central modernist architectsmanaged to reach leading positions after 1924 when the organization implementedshort periods for members of the board. This was preceded by a decline of the organizationduring WWI and a self-eroding process for ideas of traditional city-planningwhere modernist architects challenged them with effectivity-arguments thatclamed modernist ideas were scientific and traditional ideas outdated.