Londoners are being given the chance to design their own "Lego"-style Olympic Park to decide the best use for the site after the Games are over.

From tomorrow, the public can place small Perspex tiles representing houses and trees on a six-foot map as part of a planning exercise to decide the Olympic Park's legacy.

As part of the Lea Valley festival this weekend, people will be able to say what they think should be done with the 246-hectare site when the 2012 Games are over.

Concern is growing about the Games' legacy. The lack of a long-term user for the £525 million Olympic stadium is a key problem identified in a report published today. The National Audit Office said unless a tenant was found soon costs could rise again.

The 80,000-seat stadium, to be built by Sir Robert McAlpine, will be reduced to a 25,000-seat athletics venue after the Games. Organisers have been in talks with rugby and football clubs over a possible shared tenancy but so far no deal has been done.

The report by the spending watchdog said "continuing discussions on venue legacy use ... could affect the assumptions underpinning the ... budget and specifications agreed with contractors".

The Perspex map is part of a consultation intended to determine the use of the whole Stratford site.

Held in marquees throughout east London boroughs the public, armed with coloured tiles for houses, GP surgeries, hospitals, schools, shops, allotments and play areas, will decide their locations on a map.

Thousands of people are expected to take part in the consultation which will run from tomorrow for several weeks.

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