Here is my reply to your query:
Considering the facts mentioned by you that for FY 2020-21 you can save Rs.15K if you choose the new tax regime and your house property will be completed in the FY 2022-23 meaning thereby the benefit of interest on loan will be taken only from FY 2022-23, following are my suggestions:
1. For FY 2020-21 you can choose the new tax regime and save the tax as mentioned by you.
2. Once you received the possession of the house, you can then choose the old tax regime for that Financial year and take the benefit of interest on loan.
Also, to clarify that old tax regime will end in the future but as of now there is no such prediction that when this will end or some other regime will come to replace this old regime.
A salaried taxpayer can opt-in and opt-out every year. That means you can choose the new tax regime in one year and choose the regular tax regime in another year. A non-salaried taxpayer has to choose the new regime at the time of filing the tax return. They need not declare or intimate their choice to anyone at any time during the year. However, a non-salaried taxpayer cannot opt-in and opt-out of the new tax regime every year. Once a non-salaried opts out of the new tax regime, they cannot opt-in again for the new tax regime in the future.
Thanks and Regards