One of Brighton's oldest mansions, a dilapidated home that was built in the 1860s, has been heritage listed – possibly blocking plans by an exclusive girls grammar school to knock it down and build an indoor sports centre.

Firbank Grammar applied to demolish the dwelling – known as The Pines – and build the sports centre, which was to cost $2.65 million.

The Middle Crescent, Brighton, house that Firbank Grammar bought in 2014 for $3.25 million. The school proposed knocking it down to build an indoor sports centre. Credit:Darrian Traynor

But around 30 residents had opposed the plan, and a heritage building expert asked the state's heritage authority to protect the property.

Just before Christmas, the Heritage Council ruled that the building had to be kept.