Londoners will get a chance to design their own “Lego” London Olympic park tomorrow using mini Perspex tiles for houses and trees in one of the biggest public planning exercises ever held.

Vast marquees are to be set up in boroughs in East London this weekend with huge plywood grids allowing locals and passers-by to decide what they want most in the 246-hectare (608acre) site after the 2012 Games are over, The Times has learnt. Armed with a series of coloured tiles for houses, allotments, play areas, GP surgeries, hospitals, schools and shops, people will decide where they should go on a 6ft-wide map.

But they will also be able to use “wildcards” representing more exotic ideas such as a zoo, an Olympic “Eye”, bowling alley, go-kart track or spa.

The events, starting at the Lea Valley Festival, will continue over the next few weeks to allow thousands to take part. They are part of a longer consultation held by the London Development Agency to decide what should be left in the Olympic site.

Olympic organisers are determined that London will not suffer the same fate as Athens where little advance planning meant the venues and park were poorly used for three years after the event.

In a critical report today the public spending watchdog has cautioned that unresolved issues about the postGames use of the 2012 venues pose a “formidable” challenge to Olympic preparations.

The National Audit Office says that the uncertainty over funding for the £1 billion athletes’ village, the single biggest project on the site, and spiralling security costs threaten the £9.3 billion budget.

It urged Olympic chiefs to keep a “firm stance” on costs because of the economic downturn and to present a detailed legacy plan for the park.

Jason Prior, from EDAW design consultancy, one of the masterplanner teams, said that it was vital that the community was consulted at the earliest possible moment so they could highlight local needs. “Bits of the community don’t work without proper engagement, particularly where you have socially disadvantaged groups and a cultural mix.”

On Saturday teams of three to six people, helped by planners, will be able to design a blueprint, like an Ikea kitchen, which will then be photographed and filed.

The teams will be picked by the planners and groups of friends can join in for 15 to 30 minutes.

The files will then be studied by the three planning companies and the most common requests and designs are likely to be included in three templates that will go out to formal consultation in September. The best design will then be put forward in a planning application next year.

People all over the country will also be asked to send in their suggestions online about what should remain in the park. These will also be considered for the final three templates.

The amateur planners will be given a blank cheque. Although more than £9 billion has already been committed to the Olympic project, there has been no price tag on the legacy, which will include social housing, schools, hospitals and entertainment facilities.

The London Development Agency told The Times that amateur planners would have some restraints – the park would need to include 7,000 houses and 102 hectares of free parkland and the existing venues, such as the Olympic stadium and the aquatic centre, which are to be down-sized.

In addition, planners have to include industry units, office spaces, a retail area and ten public green spaces. Although they will have 54 hectares to play with, the “planners” will be able to designate some parts of the existing parkland to other areas in their designs.

Tom Russell, group director for Olympic legacy at the agency, said: “Public consultation is a vital part of our planning process for the future of the Olympic park site. These Legacy Now events have been carefully designed to draw out thoughts and views that will provide real influence for decision-making.”

The National Audit Office’s concerns echo those of David Ross, the Carphone Warehouse tycoon, who cautioned London’s Mayor Boris Johnson that a “perfect storm” of uncertainty made the Government’s £1 billion contingency fund look “inadequate”. He recommended the creation of a legacy board, at arm’s length from government.

“Whether the billions of public money now being invested in the Olympics project are seen as value for money [will] stand or fall on the legacy that London as a city inherits,” he said.