From Revenu Quebec (bold formatting is mine):

The period of eligibility for the RénoVert tax credit has been extended to March 31, 2019, for all recognized work other than work related to the construction, renovation, modification or rebuilding of a system for the discharge, collection and disposal of waste water, toilet effluents or grey water in accordance with the requirements set out in the Regulation respecting waste water disposal systems for isolated dwellings.

[...]

The initial construction of the dwelling must have been completed before January 1, 2016, [...].

https://www.revenuquebec.ca/en/citizens/tax-credits/renovert-tax-credit/