Draw the water with a vessel from the treated water storage tank and use it for watering the garden
Yet another idea would be to have a separate overhead tank for storing treated water which should have separate pipes to take the water to water your terrace and compound wall garden. This can also be connected to the toilet flush tank.
The roof has to be treated and made water proof. You can achieve this by coating your terrace with fibreglass.
A Civil Engineer’s advice has to be sought on the load bearing capacity of your house and the roof. You have to take into account the weight of the pots and pipes, you would have to install and that of the plant bed which is made of mud, sand, red sand and manure as well as the weight of the plants which will keep growing.