Some of the city's 500,000 illegal small structures are expected to be exempted from prosecution if they are proved to be safe under relaxed rules. The Buildings Department yesterday proposed allowing flat owners to retain three types of small structures - supporting frames for air conditioners, drying racks and small canopies - which had failed to obtain approval from the Building Authority. However, alterations, improvements or reinforcements may be required before the structures meet safety and dimensional standards, the authority said. The proposal is to be discussed at the Legislative Council development panel next Tuesday. The Buildings Department estimates there are 500,000 small structures illegally installed on buildings throughout the city. The most common are unauthorised signboards, sheds built on platforms or roofs, and some household facilities. The authority aims to check 1,500 buildings annually. Last year, it issued removal orders involving more than 30,000 illegal structures. To legalise structures 'of practical use to households', the authority initiated the 'household minor works validation scheme', which will allow owners to keep the structures after safety checks. The scheme is part of the 'minor works control system' the government set up to provide a simple and effective way for building owners to carry out small-scale works. The system, based on the Buildings (Amendment) Ordinance 2008, classifies all minor works into three classes according to their nature, scale, complexity and any safety risk they pose. Class I refers to relatively complicated minor works like internal staircases connecting two floors; Class II are less complex works like repairs done on external walls and Class III covers small-scale works mostly carried out in household settings such as erection of support frames for air-conditioners.