Cassiobury Infant and Nursery School has proudly unveiled its new building, giving it enough room to house an extra class.
The Bellmount Wood Avenue school was earmarked by Hertfordshire County Council for expansion, and for the past year has taken on an extra form of entry.
The new building means the school can be permanently expanded from two forms of entry to three, enlarging each year to 90 pupils.
Jeremy Alden, school governor, said: "We are over the moon with what it’s done for the school, it has dramatically changed the face of the school and will allow us to grow.
"We held a formal opening on Friday and managed to gather lots of old friends and the former headteacher and governors.
"It was a good team effort, a lot of hard work for the staff and teachers."
The new building is the same size as the school’s original building, and houses classrooms for nursery and reception.
There is also an activity room, community room and a kitchen for older pupils to use.