Where to park a bike during a year for free ? (in the Netherlands?)
Probably your best bet if you want to park them is to meet someone with a large shed who'll put the bikes up for you.

If you make the finance/convenience tradeoff I think selling the bikes after your stay and getting new ones when you return might be a better option, but I understand the convenience of having the same bike.

I can think of several options that don't require you to find such a nice person but they all have their drawbacks:


Park them somewhere on the street. This would work for an EUR 50 bike, just fix them to something with a good lock somewhere out of the way. However, the dutch climate won't be nice to your bike.
Park them in any public bike parking area near a big train station. Most of them have a roof these days, and they are free. However, in the bigger cities and all university towns you're not allowed to leave your bike for more than a few days at most, and this is actively policed. So chances are your bike will be removed under this rule. If this happens you have a few months to pick it up from the depot before they sell it off.
Park them in a "private" bike parking area near a big office building. This might be a reasonable option if you are willing to do a bit of sneaking to get in. Most of these bike parking areas have room to spare, so two exta bikes won't bother anyone, and if you park far away from the office entrance no-one will notice the bikes standing there all the time. Downside is that these buildings are usually not in very central locations. You could even ask the concierge if he'll let you park the bikes for a few months (with the risk of him turning you down where he wouldn't have noticed the bikes if you hadn't asked).


In all cases the property owner or the local city government is within its rights to just remove your bike if it is deemed to be in the way or a wreck, which is always a risk for a EUR 50,- bike.