It's possible to transfer the deduction balance to other objects only within acquisition of an apartment after 2014 This won't work with mortgage interests, as this deduction is provided only for one object.
When you can apply for the deduction
Option 1 - Within the next year after receiving the documents of title (for a sale and purchase agreement - this is the Unified State Register of Immovable Property, for a shared construction participation agreement - an act of transfer and acceptance).
Whenever you purchased an apartment or other housing, upon obtaining the rights of ownership, you can claim a tax refund after two years, or after ten years, as you choose.
But at the same time, the tax can be refunded only for the last three years. For example: you bought an apartment in 2016, and decided to apply for a property deduction only in 2021, 5 years later. This means that you can file a tax return for 2018, 2019, 2020. And further, if you have an unused deduction, for 2021 and subsequent years.
Pensioners can return personal income tax at once for four years: for the year in which the purchase was made, and for three previous ones.
Option 2 - Apply for the deduction to the employer in the year of the purchase and registration of documents for the property. This can only be afforded by citizens working under an employment agreement. Please note that the date of applying for refund does not affect the deduction allocation rule from 2014