I have savings that I want to put into a deposit to buy a house in the Netherlands. I heard that saving above 21K is taxable at 1.2%. Is this true or is it exempted if you have the 30% tax ruling?
It's true that savings above 21K are taxed at 1.2%, this is called “box 3 inkomsten”. The logic is that savings and other assets are assumed to bring you 4% interest and you have to pay 30% tax on this (fictional) income, no matter the effective interest rate. That's the general case.
Now, there is indeed an exception for people who benefit from the 30% ruling. You can choose to have a “partial non-resident status”, in which case investments and savings (assets in box 2 and 3) are taxed following the rules for non-residents (whereas wages and other box 1 income are still taxed according to the regular rules, minus the 30% rebate). It means that you are only taxed on your assets and income sourced in the Netherlands. Additionally, there is an exception for bank accounts, life insurance policies, etc. so you would really only pay taxes on real estate and ownership of Dutch companies.
In any case, this is apparently something you have to apply for explicitly so it would make sense to hire a tax advisor to go over this and help you with the formalities. You are not supposed to simply put the money aside and do nothing, as far as I know.
Finally, the box 3 income tax is a tax on net wealth, and not directly a tax on money held in savings account or elsewhere. So you can also discount (some of?) your debts before computing the basis for the tax. I have no first-hand experience with this so I don't know how it works in details but that's something else to consider.