Newland House School will bring together its two Waldegrave Park buildings as part of major development plans for the Twickenham independent school.
It received planning permission to create a new building on the site of a residential property next to the main school building, which it acquired in 2005.
Since 1974, the school's pre-prep, for its youngest students, has been detached from the prep school in an adapted Victorian property.
As well as creating a new state-of-the-art combined learning environment, it will offer additional places reception in September 2016 onwards and will cease to offer entry to year 3 via the 7+ assessment.
Headteacher David Alexander said: "Uniting the two parts of the school will facilitate a natural and seamless transition for younger pupils as they progress through the school and our whole school community will have an opportunity to enjoy the shared benefits that unity will bring.
"In the new pre-prep, our pupils will flourish in a sustainable and expertly planned learning environment which allows us to explore innovative learning styles."
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