In what could render 150,000 flat owners homeless, the Bombay High Court has ruled as ''illegal'' all buildings constructed on a piece of the city's 120-acre land, falling under the category of forestland. A division bench of Maharashtra Chief Justice Swatenter Kumar and Justice S C Dharmadhikari held the construction of several buildings in the suburbs of Bhandup, Mulund, Borivli, Dahisar and the Thane belt as illegal.