A close-up of this window to their apartment was shown right after many Milwaukee episode's intro was shown,
where one of the cast members said: Laverne & Shirley is filmed before a studio audience.
The window filmed as being their basement-apartment window is one of the two lower windows next to the steps, probaly the one nearest the steps.
This building is still located on Washington Square, on the back lot at Paramount Pictures Studios in Hollywood, California.
This is NOT where any episodes were filmed; this building was ONLY used for the opening credits, and a few outside shots during certain episodes.
The actual episodes were filmed on Stage 20, at the front of Paramount Pictures Studios.
Paramount Pictures Virtual Tourdate unknown, but probably pretty-recent Hollywood, California Paramount Pictures Studios Back lot Laverne & Shirley's apartment window John Mark Motyer, CD