Rainfall

 Average annual rainfall Paris region

637 mm per year.  Area of Roof  126.68 sq m (Inkscape)

annual volume of water on roof  0.637 x 126.68 = 80.7 cu m. 0 80 000 litres/year

Low flush toilet say 6 litres per flush.  Ave 2 people 5x per day = 60 litres per day 

= 60x365 = 21 900 litres per year

https://www.gammvert.fr/2-1360-outils-de-jardinage/2-5031-arrosage-recuperateurs-deau/3-3592-recuperateurs-deau/p-43196-cuve-de-recuperation-deau-enterree-garantia-jardin-flat-confort-5000-l#ref=3592-3

5 000 litre underground tank kit with pump 3100 euros

https://www.cuve-shop.fr/cuve-a-eau-de-pluie-aerienne-ronde-10000-litres-diametre-240cm.html

10 000 litre tank