The three storey 912 square metre Victorian building at 259 William Street is expected to fetch up to $6 million when it goes to auction at the end of March.

The building is in the heart of the legal district, next door to the Magistrates Court, opposite the County Court and a short walk from both the Supreme and Federal courts.

Title deeds show it was purchased in 1969 for $104,000 and is jointly owned by Kidson Nominees and Peter O'Bryan Nominees

John Galbally started the firm in 1935 and was joined by his brother Frank in 1948. Frank Galbally joined forces with Peter O'Bryan in 1965 and began a formidable partnership.

Galbally O'Bryan will lease the building until 2020.

CBRE agent Josh Rutman, who is handling the deal along with Tom Tuxworth, Lewis Tong and Killen Thomas's David Marks, said it was a rare opportunity to own a whole building on the legal strip.

"You can count on one hand the number of freehold buildings worth under $10 million along William Street. Most buildings are large office buildings, barristers chambers or strata titled floors," Mr Rutman said.

The last building to sell under $10 million on the strip was the nearby Metropolitan hotel which fetched $8.61 million last year.

Strata office is popular however, especially in the legal district. A coveted whole fifth floor office at 530 Lonsdale Street, formerly known as Lonsdale Chambers, is for private sale.

Offers for the 777 square metre office belonging to Michael Sifris, SC, now a Supreme Court judge, close on Thursday.

Also going to auction this Friday is 536-567 Little Lonsdale Street, a refurbished warehouse on the corner of Manton Lane.