Two companies stand to make a combined total of more than £14m if an appeal to build a £30m floating homes scheme on the Surbiton filter beds is successful.
Developers Cascina and Hydro Properties could make as much as £10.76m in profit.
But they are relying on a decision by Kingston Council to refuse its 64-home and 92-berth marina development to be overturned by a planning inquiry taking place this week.
If the scheme does go ahead, the company has agreed to pay former site owners Thames Water at least £3.72m in “overage” payments, having originally paid the company just over £1m for the land.
The figures were revealed in confidential viability reports made public shortly before a five-day hearing into Hydro’s scheme began at Surrey County Council’s headquarters in Penrhyn Road, Kingston.
The hearing will decide whether Kingston Council was right to refuse permission back in January, on the grounds it is an inappropriate development on protected Metropolitan Open Land.
The inquiry began on Monday, with lawyers for Cascina and Hydro, Kingston Council and opposition group Friends of Seething Wells setting out their positions.
Planning inspector Leslie Coffey is expected to give a decision in January or February
Hydro's Christopher Katkowski
For Hydro, Christopher Katkowski described his clients’ project as “a wonderful, valuable and interesting place for the public to visit”.
He said: “The proposals are well thought through and will develop a range of tangible, worthwhile benefits for the community.
"For the first time this site will be open to the public."
Kingston Council's James Strachan
But James Strachan, representing Kingston Council, said: “The much-vaunted leading edge technology in making these homes ones that can float is not unique.
“The reality is more banal.
“This site is being used for little more than a showcase by a company for a product which is not needed or required in this Metropolitan Open Land location.”
Simon Tyrrell giving evidence
Simon Tyrrell of Friend of Seething Wells went further in his opening statement, calling Hydro’s plans “deliberately misleading” and incurring the wrath of Mr Katkowski, who branded his comments defamatory.
The viability reports came to the fore on Tuesday when Surbiton resident Brian Wilson, appearing as a witness for the Friends of Seething Wells, questioned how the developers could call the project an “enabling” scheme beneficial for the community when its projected profit was so high.
Speaking after his appearance, Mr Wilson told the Surrey Comet: “I said it was either a profit development or an enabling development, and that you can’t have it both ways."
Former Kingston Council leader Roger Hayes and other members of the public alongside Simon Tyrell (left)
Former deputy council leader Barry O'Mahony
The inquiry, chaired by planning inspector Leslie Coffey, has heard from several other witnesses including Kingston and Surbiton MP Edward Davey, Kingston Council leader Liz Green and councillor Barry O’Mahony.
It concluded yesterday, with a decision expected in late January or early February.
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