The city is implementing the law in three phases. Downtown Toronto is first on the list because the city has a combined system in which the water coming off roofs may mix with the sewage system which can become a problem with a large downpour because sewars can become overloaded, increasing the risk of basement flooding and releasing polluted rainwater into our local waterways.
There is some good information here
You may be able to apply for an exemption, but you’ll have to apply in advance. Click here.