US private equity firm Silver Lake is buying Zoopla and PrimeLocation owner ZPG for £2.2 billion.
The deal will give the Daily Mail publishing group a £642m windfall for cashing out of online property portals.
Along with rival Rightmove, ZPG dominates online searches for property sale and rentals in Britain, with Zoopla's sites carrying listings for nearly 15,000 estate agent branches.
Analysts and bankers said the ZPG sale was likely to be used as a price benchmark for other mergers and acquisitions involving such portals and comparison websites.
Launched in 2007 by Alex Chesterman, who was also behind the LoveFilm video-on-demand service acquired by Amazon.com, ZPG was floated in 2014 and its shares have since risen by 69%.
Under the terms of the agreed deal, each ZPG shareholder will get 490 pence in cash, a premium of 31% to its closing price yesterday, Silver Lake said.
Newspaper owner Daily Mail and General Trust, which merged its property portals Findaproperty and PrimeLocation with ZPG in 2012, will receive £642m for its near 30% stake.
If the deal completes, DMGT's returns from online property will total £890m, more than 14 times the cost of its original investments, it said, after its shares rose more than 9% to a 16-month high.
Meanwhile, ZPG's second biggest shareholder, hedge fund Lansdowne, declined to comment on whether it supported the bid.
Silver Lake, with around $39 billion in assets under management, said ZPG, whose websites and apps attract more than 50 million visits a month, was a great growth technology story.
Shares in ZPG, which also owns utility price comparison website uSwitch, were up about 30% to 486.4 pence after ZPG directors said in a joint statement they considered the terms of the deal "fair and reasonable".
The directors of ZPG, who own 1.16% of the firm, also backed the takeover, which analysts said showed that buyers were prepared to pay "punchy multiples" for high quality assets.
They also said the deal shows UK consumer-focused online companies are increasingly seen as attractive takeover assets.
ZPG itself made a £460m takeover offer for Gocompare.Com Group, which the price comparison website operator rejected in January.
And earlier this year, German publisher Axel Springer made a nearly £125m investment in UK online real estate agent Purplebricks.
