A Sydney property developer is set to gain more than $1 million for selling a home he found empty and rented out for two decades.

Key points: The house was left empty after its last tenant died in 1998

The house was left empty after its last tenant died in 1998 The renovated home sits on 498 square meters and has a large back garden with a roofed balcony

The renovated home sits on 498 square meters and has a large back garden with a roofed balcony A Supreme Court judge found Bill Gertos had "assumed the position of a landlord"

The NSW Supreme Court last year awarded Bill Gertos the home under "adverse possession", or squatting laws, despite a challenge from the relatives of the last listed owner.

The house was put on the market by Mint Property Agents, who refused to provide comment to the ABC.

Based on the estimated suburb median, the property is expected to sell for around $1,425,000 but several real estate agents told the ABC it could be worth an estimated $1.6 million.

Mr Gertos last year told the court he came across the three-bedroom Ashbury home in 1998 and claimed it was open and "the rear door was off its hinges and placed to the side".

The house had sat empty since its last tenant died the same year and Mr Gertos said he found it while visiting a client on the same street.

He said he changed the locks and spent more than $140,000 renovating and repairing the house before renting it out.

The family of the home's registered owners challenged Mr Gertos in the NSW Supreme Court when he applied to be named the owner of the land under the Real Property Act in 2017.

But Justice Rowan Darke found Mr Gertos had sufficient evidence to show he invested money into fixing the home, paid taxes on it and leased it to rental tenants.

"In essence, Mr Gertos succeeded in taking and maintaining physical custody of the land, to the exclusion of all others, and he has assumed the position of a landlord," Justice Darke said.

The renovated home sits on 498 square meters and has a large back garden with a roofed balcony.

The daughter and two grandchildren of the registered owner, Henry Thompson Downie, disagreed the home was in poor quality when Mr Gertos found it and claimed the property was never abandoned.

The family said they were unaware they were entitled to the property until they were contacted by police in 2017.

Neighbours said they were unhappy about the decision and it was unfair to the relatives of the last owner.

In 2015, Mr Gertos was ordered to pay $250,000 worth of fines after illegally demolishing a heritage building in Sydney.