Long story short: not at all. The value of a home is determined by the surface area that has a residential code/ destination. A storage room doesn't have a residential destination and is therefore not seen as usable living area.
The NVM the biggest real estate association in the Netherlands, makes a clear distinction in this regard (see link):
Usable area, also known as living area, is the sum of all usable areas within a home, such as the kitchen, living room, bathroom, and bedrooms, etc. These areas are all heated, well insulated, fireproofed, and accessible within the object, for example via a hallway or stairs within the house itself.
For this reason, a separate storage room belongs to the so-called "additional indoor spaces." In Amsterdam, these storage rooms are usually located on the top floor or in the basement of an apartment building. Because storage areas are sporadically used or visited, they are usually not heated and little to no daylight enters.
They are therefore not or hardly insulated, and they are not up to building code or eligible for obtaining a residential destination/code. It is therefore prohibited by law to use a storage room for anything other than to store things.
This is also the reason why we are applying for a permit to turn a storage room into a real living space.
If we look up two homes in the same district on real estate website Funda, with the same number of square meters, one with and one without storage, we see that they have a similar value. The square meters intended for residential use are what determine the price with the NVM broker's measurement.
A house on your own/private land or with 'Erfpacht' leasing the land? The ground lease makes no difference to the value of the house, even though the owner of a house with a ground lease needs to pay a substantial amount every year. The number of square meters of living space or user space is leading.
For more information, visit the NVM Netherlands website.