The firm, set up by Evan McPherson and his partner Samantha Homes, is on the site of a former coalmine, East Bristol Collieries, which closed in the 1920s.
Hanham Pit opened in 1906 near to the Avon, but closed in 1926 when the price of coal tumbled in the 1920s after the high demand in the First World War tailed off and the broken strata of seams proved unprofitable.
Reviving the historic site using the former winding engine house, East Bristol runs monthly auctions, quarterly military and toy sales as well as timed online auctions from its base in Hanham, Bristol.
It has agreed to lease space in the next-door warehouse unit to expand and will nearly double its saleroom and storage space in the coming months.
Following the expansion it will operate from 10,000 sq ft and the company will build a mezzanine level to make the new space two floors.
The firm employs nine people and has been specialising in Mid-century and Design-led sales as well as focusing on toys.
McPherson said: “We have some great young people working here who are trialling and testing different sales, like our Lego only sale.”
He said the firm sells around 75% of its lots to online buyers and has an 82% average sell rate and an average lot value of £250-300.
In 2016 the firm was in the top 10 for the highest average bidder registrations per auction from thesaleroom.com with 11,667 across 26 auctions.