The lender is also increasing the maximum loan to value (LTV) on its part and part repayment products and updating affordability calculators.
The changes will come into force on 15 April, with cases needing to be submitted by 9pm on 13 April for acceptance under existing criteria.
Any full mortgage applications submitted from 6am on 15 April, or where a material change is made to one originally submitted before 9pm on Saturday 13 April, will be assessed using the new policies.
Term, age and LTV changes
For residential capital and interest repayment deals, Santander is extending its maximum mortgage term to 40 years from 35 years.
However, for interest-only where the repayment is by endowment or investment, it is reducing the term from 35 years to 25 years.
Interest-only where the repayment is by sale of property remains at 25 years.
For part and part repayment deals, Santander is increasing the maximum LTV to 85 per cent from 75 per cent, with a maximum 50 per cent on interest-only.
For buy-to-let lending, Santander is extending its maximum term to 40 years from 25 years and also increasing the maximum permitted age at maturity to 85th birthday from 75th birthday.
The maximum LTV offered on buy-to-let mortgages remains at 75 per cent.
The lender said it was also making some changes to its affordability calculations.
These are in the form of annual updates to household expenditure figures and changes to income tax bands and national insurance to align with the 2019/20 tax year.
Its BTL self-financing calculation which runs in the background is also being updated with the buy-to-let calculator reflecting those changes.
Santander head of channel development Graham Sellar said: “By increasing both the maximum term and maximum lending age on our buy-to-let mortgage range, we’re hoping to help landlords reap the benefits of their investment for longer.”