How to allocate assets between asset types and tax-benefit accounts?

Your situation with the tax-benefit pension account is far better than mine.

I have access to a tax-benefit pension saving scheme where it is not possible to terminate the contract at will, even if returning all tax benefits. The tax benefit applicable to this scheme which I can use is basically that I get a tax discount for the saved sum, but must later after retirement pay taxes of the entire withdrawn sum and not just the profit. So, in practice you get 1/(1-taxrate) * (1+p)^N * (1-taxrate), i.e. there's effectively no tax (unless the tax rate changes in which case the two tax rates might not be equal).

Because of this, I didn't start massive savings to this tax-benefit account. Only 0.13% of my funds are in this tax-benefit account (and I can't withdraw the 0.13% initial payment I made until I retire).

Your situation seems far better. If I understood the question correctly, you can any time withdraw all of the saved money, along with any capital gains, just you have to return all tax benefits. So it's not any worse than making regular stock investments, apart from the two problems:

1. You must withdraw all (by terminating the contract) or nothing, you can't withdraw half of the money
2. Stock investments are limited to 40% (and this is the really big issue, as anyone sane saving for pension will choose a very very high stock amount)

So if you really want to save to this tax-benefit account, you should use 40% stocks and 60% bonds.

Where I live, bonds have a very slightly negative yield. But let's assume for simplicity that the yield of bonds is zero. The yield of stocks is about 2% economic growth + 2% inflation + 3% dividend yield or 7%. So 40% stocks and 60% bonds would give 2.8% yield. That would be subject to some tax benefits. Let's assume the tax benefits are that there are no taxes (as you didn't specify the full taxation in the question). So you get 2.8% nominal yield, no taxes.

By investing 100% to stocks outside of the tax-benefit account, you get 7% nominal return before taxes. Let's assume taxes are 30%. Let's also assume taxes are paid at the end of the investment and not gradually as the values of the stocks rise. So in 20 years you have (1.07^20-1) * (1-0.3) + 1 = 3.0088 times the initial investment, or 3.0088^(1/20) = 1.0566 per year. So the yield even after taxes is 5.66% per year.

Someone could say I'm not comparing apples to oranges by comparing 60% bonds / 40% stocks portfolio with 100% stocks portfolio. Anyway, I don't think this is a big issue because if you only can terminate the entire savings contract, you should be prepared for a long-term investment in the pension scheme anyway, so you really want 100% stocks there.

So, I will propose don't use the pension saving scheme because of the "at most 40% stocks" limitation. Even if you wanted to invest let's say 25% to bonds and 75% to stocks, you could save 25% to bonds in the pension saving scheme, 16.6667% to stocks in the pension saving scheme and the rest to stocks outside of the pension saving scheme to gain 25%/75% allocation, but then the part you may want to access (i.e. bonds) is inside the scheme and you can't withdraw any of it without terminating the whole contract. So doesn't make any sense.

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However, the big question is do you want 100% stocks or let's say 25% bonds / 75% stocks. That depends on the time horizon. You mentioned buying a house. If you are planning to buy a house in the next 5 years, it might make sense to have a portfolio based heavily on bonds.

However, before you do so, estimate the risks and benefits of buying a house and living in it as opposed to renting.

For example, I have demonstrated here that house prices can and do fall 80% in real terms just like stock prices. So the risk of a well-diversified house investment (hundreds of different kinds of houses) is the same as it is for stocks. A one single house will have higher risk than a well-diversified house portfolio. So, if stocks yield 7% when well diversified you probably want at least 9% yield from the house investment to get any reward of the large risk you take to buy an individual house and not a well-diversified portfolio of houses.

Where I live, the management + heating fees of an apartment are about 4 euros per square meter per month or 48 euros per square meter year.

The construction costs are 2500 euros per square meter. It is well known that to maintain a house valuable during hundreds of years, you need to invest the entire construction costs during a 50-year period or 2% of construction costs per year in larger projects like re-doing the plumbing. So you should be prepared to spend 50 euros per square meter per year.

However, even if an apartment building can be constructed for 2500 euros per square meter, the land below it and the building permit are so valuable that apartment buildings sell generally for 4500 euros per square meter where I live. So, to get 9% nominal return or 7% real return (for 2% inflation), you have to get a benefit of 315 euros per square meter per year. But that doesn't take into account the fact that owner-occupancy is not taxed, so with a 30% tax rate, 9% nominal return before taxes is 6.3% nominal return after taxes or 4.3% real return. So let's say 193.5 euros per square meter per year is enough as opposed to 315 euros per square meter per year.

So, if your rent is more than 193.5 euros per square meter per year + 50 euros per square meter per year + 48 euros per square meter per year, or in total 291.5 euros per square meter per year, or as a monthly sum of 24.3 euros per square meter per month, it makes sense to buy a house. Otherwise it makes sense to rent.

Guess what? I don't pay 24.3 euros per square meter per month for my rental property. I pay 11.6 euros per square meter per month.

So for me it doesn't even remotely make sense to purchase a house and living in a house I own. It makes far more sense to rent and invest into stocks.

Do a similar calculation for you. If living in a house you own makes sense, then you might prefer a large share of bonds in case you buy a house later, but otherwise, do prefer a portfolio heavily biased to stocks (let's say 90% stocks at least).

And before investing into stocks, don't forget to first maintain an emergency fund large enough. The stocks percentage should be calculated only from your investments that are not part of the emergency fund.