Shareholders of the estate agent-backed hub will receive 110p per share, which is a 56% premium to the closing price of the business on 18 October, the day before the announcement. 

Aim-listed OnTheMarket, founded in 2013, competes with Rightmove and Zoopla in the house sales market. 

The UK portal posted a narrowing full-year pre-tax loss of £200,000 in July, down from £1.2m a year ago. Sales lifted 14% to £34.4m. 

But it added that estate agents cancelling contracts with the business were “higher than expected”. 

CoStar, founded in 1987, has a market value in New York of over $33bn, adds that the move offers “an attractive strategic entry point for CoStar into the UK residential property market”.   

The US firm’s websites attracted around 280 million visits in September, and include Homes.com, second-largest US residential marketplace. 

CoStar adds: “The acquisition will be an important step in the expansion of CoStar’s Homes.com residential network not only in the UK, but across Europe.” 

CoStar founder and chief executive Andy Florance says: “CoStar has a strong track record of acquiring, investing in and building online property marketplaces into leading positions, including LoopNet, Apartments.com, and most recently Homes.com.” 

“We are excited to combine CoStar’s resources and leading marketplace experience with OnTheMarket’s agent-focused culture to create a leading player in the UK marketplace and provide increased choice and higher quality offerings for consumers.” 

OnTheMarket chairman Chirs Bell adds: “The offer from CoStar recognises the quality of OnTheMarket and the significant potential of the business, while offering shareholders an attractive opportunity to realise their investment at a substantial premium to the prevailing share price.”