Most famous beaches are accidents of geology. Strandbad Wannsee is a policy. It is a piece of Weimar-era social democracy poured into sand and clinker brick, a deliberate machine for giving the industrial working class of Berlin a seaside they could reach on a tram ticket. The singular fact of this shore is not the water and not the view. It is that a landlocked lake beach became one of the largest inland lidos in Europe because the city decided its poorest citizens deserved a coast, and then built one, importing the very sand from the Baltic to do it.

The people's beach. By the mid-1920s the shore was already busy, drawing some 900,000 visitors a year to a patchwork of fenced men's, women's and family sections left over from 1907. Berlin's city building councillor, Martin Wagner, wanted something on a different scale. Wagner was not a minor functionary. Appointed Stadtbaurat of Greater Berlin in 1926, he was one of the driving forces of Weimar modernism, the planning head behind the horseshoe-shaped Hufeisensiedlung housing estate now on the UNESCO World Heritage list, and a Social Democrat who believed public works were how a democracy paid its citizens back. He conceived the Wannsee lido as a Weltstadtbad, a bathing works for a world city, sized for the masses who could not afford the North Sea resorts.

The architecture. The buildings were the work of Richard Ermisch, and they are deliberately plain. Ermisch built in the New Objectivity idiom, Neue Sachlichkeit: function first, no ornament, the look coming from the materials themselves. Long two-storey blocks of yellow clinker brick on a steel frame line the approach, holding changing rooms, kiosks and terraces, arranged by Wagner so that crowds could flow from the S-Bahn to the sand and back without crushing. It is one of the largest works of Weimar public architecture built purely for leisure, and it was aimed downward, at the ordinary Berliner, not at a spa clientele.

The sand. The most literal proof of the ambition is underfoot. The Wannsee is a lake, a bay of the Havel, and its natural bottom is not a bathing beach. So the city gave it one. The 1,275-metre strip was laid with sand carried in from the Baltic coast at Timmendorfer Strand, wagonload by wagonload, a coastline trucked inland by rail so that people who might never see the sea could lie on it. The beach is 80 metres deep and holds around 12,000 bathers when full. Little about it is accidental; almost all of it was decided and delivered.

The Depression halved it. The plan Wagner and Ermisch drew was bigger than what stands. A budget of 5 million Reichsmark was to have bought several more of the two-storey halls, a large circular restaurant, a pier with a café, a marina and an open-air theatre. The Wall Street crash and the Depression that followed cut the money roughly in half. Only the northern axis was completed, four buildings instead of the full ring, and the grander water architecture was never built. What you walk through today is a half-finished ideal, an ambitious democratic gesture caught mid-sentence by an economic collapse. It still opened to about 1.3 million visitors in 1930.

Then the state turned on it. The same public that the lido was built to serve was redefined within a few years. Wagner, as an SPD man, was pushed out: expelled from the Deutscher Werkbund in 1933, gone from Germany by 1935, teaching planning at Harvard by 1938 with Walter Gropius's help. The beach he built stayed, but its openness did not. Under the Nazis, Jews were excluded from public bathing facilities, a bar made explicit in 1938; the antisemitic signs came down temporarily only to keep the beach presentable to foreign visitors during the 1936 Olympics. A place designed as a democratic commons was converted, without changing a brick, into an instrument of exclusion.

The afterlife. The lido outlived all of it. Between 1951 and 1955 it had a stranger danger, stray bullets from the adjacent American Keerans Range that struck swimmers and forced a closure in 1952. In 1951 it also became a pop song: "Pack die Badehose ein," sung by eight-year-old Cornelia Froboess, made a Wannsee swimming day into a hit and a stock German phrase, and was banned regionally in East Germany as covert advertising for a West Berlin lake. Restored in the mid-2000s and protected as a listed monument, it still runs as a public bath under the Berliner Bäder-Betriebe, day ticket a few euros. What makes Wannsee worth a page is not that it is big or old. It is that it is a built argument, a Weimar promise that a city owes its people a coast, delivered in Baltic sand and then made to carry a century of German history it was never designed for.