The building Christian Scientists call the Mother Church was not built all at once. The first structure on the site at the corner of Falmouth and Norway Streets in Boston was a comparatively modest granite edifice in the Romanesque style, dedicated in 1894, seating roughly a thousand people. It was already the second home of the congregation Mary Baker Eddy had organized in 1879; the church needed a permanent building of its own once membership had grown past what rented halls could hold.
Growth did not stop with the new building. Within a decade the original edifice was full past capacity on the Sundays and Wednesdays the denomination's branch churches everywhere still hold as their standard meeting days, and the congregation resolved to build again rather than relocate, adding to the existing structure instead of replacing it. The result, dedicated in 1906, is the domed Extension that most visitors now think of as the Mother Church: a much larger auditorium, more than four times the seating of the original building, built directly onto it so that the small granite church of 1894 still stands, physically attached, inside the footprint of the larger complex that grew up around it.
The two buildings together, plus the later Christian Science Publishing Society building and the reflecting pool, form what the church calls the Christian Science Plaza, still an active place of worship rather than a preserved monument, and still the constitutional head of every branch Church of Christ, Scientist in the world.