What point on the main land of The Netherlands is furthest away from any buildings?

Possibly het Verdronken Land van Saeftinghe (Saeftinghe's Drowned Land), depending on your exact criteria. The closest municipality is Emmadorp, which is around 5-6 km away from the furthest point in Saeftinghe.

The closest town is Graauw, which is another 2 km further. The closest actual city is Hulst (which is still rather small), which is 10 km away from Emmadorp.

Why I picked Saefthinghe is not just because of its distance from civilization, but also because of how it shows how insignificant our civilization is. Under the sand in this swamp-like area is buried the town of Saeftinghe. Every few hours, with the rise of the tide, everything there gets washed away. Nothing can be built there, nothing can persist, except a few brave plants. Legend says, on foggy days, you can still hear the town bells call for help from the rushing tide that ultimately washed the town away.

Being the farest away from any build up area while still being on open land that remains land 24/7, I bet it is in one of the polders, with your best bet on the area between Lelystad, Almere and Zeewolde.
The distances are about the same as those in the 'Verdronken Land van Saeftinghe' answer, the land is always available, not claimed by sea/water at any time.

Where you might feel the most remote from buildings, the few forrests we have. I often walk in the Dunes, the area between Zandvoort and IJmuiden is best for this, there is the bigger area between Wijk aan Zee and Bergen where you find a few houses but once there are a few trees between you and them you forget about them.

I think there are also areas in the bigger forest reserves, like de Hooge Veluwe and de Veluwe zoom, but I am less familiar there.

It being the Netherlands, you may be away from houses, you will not be away from people and nowhere you will be far away from human activity as almost all nature in the Netherlands has been build and is maintained.

For the furthest away from built-up area, I think the answer is a very unsatisfying one: the middle of the Afsluitdijk. There are no towns or villages along it. You will manage to get about 15km away from any significant group of houses.

You could go for a stroll for a mile, or even a few miles, but you probably won't find what you are looking for because you will be right next to a motorway.

I'll give the furthest away from any building a pass as there are a few buildings along the Afsluitdijk.

If you don't like middle of the Afsluitdijk or the Houtribdijk then I would suggest the banks of the Dollard in Groningen, north of Finsterwolde. Nearest village/hamlet (Ganzedijk or Woldendorp) seems 6.5-7 km away. There are some buildings (mainly farms) closer by.

Great question. I've been wondering about a similar thing: 'which point is furthest away from tarmac'. I'm not really into geospatial software, but I guess it's possible to calculate an answer based on Open Streetmap data. If I ever get around to doing that I'll update this answer.

Based on my experience, there are a few candidates on the Veluwe. But like the other answers, I don't know enough about other areas to give you a definite answer. We'll need some math to get that. Just judging from the map somewhere in the middle of nationaal park Veluwezoom would be a good candidate.

If you want to do a walk without seeing buildings, there is a trail from Hoenderloo to Radio Kootwijk. I guess that gives you around 7 km between the buildings on each end.

The north of the Netherlands might give you better result on the between towns part, but maybe not on the between buildings part because there are usually a few farms around.