Differences between residential and buy-to-let mortgages:

An investment mortgage (or buy-to-let mortgage) works differently than a residential mortgage.

A residential mortgage is for individuals who plan to live in the property. Residential buyers can get a loan up to 100% loan to value.
A buy-to-let mortgage is for individuals who plan to buy a home as an investment. A rule of thumb is that investors can get a mortgage up to 70% loan-to-value (LtV).

You are refinancing when you switch your residential mortgage to a buy-to-let mortgage. In most cases, you must pay the penalty to your existing mortgage lender for breaking your current contract.

Requirements for a buy-to-let mortgage

You must contribute 30 % of the property price from savings or equity from an existing property.
You need to be registered and live and work in the Netherlands
You must pay 10.4 % of the transfer tax and other closing fees (real estate agent, valuation report, notary, translator, and mortgage brokerage fees).