Mortgage deduction is to reduce your income tax by 1% of your loan balance (at maximum) for next 10-15 years. The impact is huge and you should not miss it.
Mortgage deduction is applicable to a house only in Japan because the deduction is only for residents in Japan.
The following documents are usually needed to collect the necessary information:
1) Purchase agreement. It should include cost, the year that it was built, space. I think you can find all these information in purchase agreement,
2) Mortgage statement, that shows principal loan amount, interest rate and the balances of each calendar year.
3) If it is qualified as”Eco-friendly”, you need to provide a certificate that your house builder should have given to you.
Please be noted that the deduction is applicable for a second-hand house too but it has to be less than 25 years old.