Seating capacity at the Church of Mary Immaculate and St Gregory the Great is being increased from 240 to 340.
Higher attendance at mass and other services has necessitated the need for the church to be enlarged, and this has been achieved by construction a two-storey extension at the front.
Mass and other services have been held in the parish hall, a community centre and nearby church while construction work was underway.
Known to its parishioners as St Gregs, the church was rebuilt in the mid-1970s after being almost destroyed in a fire in 1973 in an infamous local case of arson.
Although many parishioners have appreciated the light and airy interior of the current church building, consecrated in 1977, it has been a struggle in recent years to accommodate all those attending the Saturday evening mass and the four Sunday masses.
Often on holy days there has been quite a crush and some parishioners were left crowding around the entrance. The extension will create a new front door with an additional seating area and gallery above.
The site in Union Street was purchased for a church in 1853, and the original building was enlarged in 1878.
For details of dates and places for services until the church re-opens go to www.parish.rcdow.org.uk/barnet