Preserving and increasing the value of existing real estate, what is important?

And with that, the house is heat pump-compatible. Additional insulation is not needed (at best the basement ceiling, if available. And if you go to the roof, you can add insulation there as well).

You need a new heating system if the old device is broken. So it's an investment anyway. And since you HAVE to install renewable energies starting next year anyway, you can also safely go for a heat pump (gas alone will no longer be allowed).

The buyer will then say to you, "Oh crap, I have to renovate everything to be able to operate a heat pump. Without insulation and new windows that will never work. I'll just reduce the purchase price by €75,000." If the heat pump is installed in five years and is sold in 10 years without having renewed or insulated the windows, there will be no discussion. It works well.

That is interesting. We don't have a basement, the top floor ceiling is insulated, and we would of course insulate the roof directly during the attic conversion.
But the logic of installing it beforehand and then having something functional, presentable, and verifiable, instead of having to convince potential buyers of something, is very understandable to me. That definitely makes sense...

Definitely the heating. The rest is mostly aesthetics and can be done quite quickly by the buyer themselves. A heat pump also works with radiators, and if you can demonstrate this directly to the new buyer, it makes a big difference. They save themselves the actual conversion and the uncertainty of whether the heat pump will work. The rest is still fresh anyway given your young construction year (roof, windows).

We sold our house after about 14 years, everything we had changed/modernized in terms of aesthetics had no real impact on the selling price. Of course, you can replace the beech staircase with something you like better, but only because you like it better and not with the intention of increasing the value. What you change and modernize now can also be outdated in 15 years and/or might not appeal to the buyer. Therefore, I would only replace the heating if necessary or if it makes financial sense for you. Maintain the property and preserve its value - yes, but modernizing now for a sale in X years doesn't make sense from my point of view.

You are right, of course the next owner might find my dream country-style staircase ultra ugly and want to tear it out immediately. It is then naturally advantageous if he sees it as fitting within his budget = as feasible because he won't have to install a new heat pump.

We would have more equity after the sale (since the value has already increased significantly – although the house, of course, is getting older on the other side) and no other liabilities, so less to pay off than with the remaining debt.

You are still arguing incorrectly. The consideration is whether to sell now and buy new. And there the high sales and purchase prices balance each other out again. You can take the old loan with you. In the future, the only question is whether you still want a loan or not. Even then, it does not matter whether the property is sold with remaining debt or not. That’s what I’m getting at. Or do you want to suggest that in old age, without child(ren), you still want to build a 200 sqm palace on 2,500 sqm of prime building land? Then of course it makes sense to have more equity. But once you are over 50, you probably won’t want to build bigger together. Just different – maybe as a bungalow. The property must fit the respective life situation.

Yes, the heating costs are manageable, I just have my doubts about the heat pump. We will have to get advice on that, but currently seems to be the worst time for a heat pump.

I can report from my own experience (house from mid-60s WITHOUT insulation). Currently with 31 (with underfloor heating) and 35 (with radiators) degrees flow temperature. Very efficient and cost-effective. There are two apartments in the two-family house without external insulation. But it must be well calculated by the energy consultant for the heating design. For your house, you can heat blindly and cost-effectively with a heat pump.

Let me summarize: You live with a growing only child in a "terraced house" by standard, of which you are already the second owners. Terraced houses are always in demand on the market – but hardly by people looking for an optimal individual planning standard, rather a pareto-optimal economy standard. This will still be the case in five years when you can change the property with untaxed "speculative gains." Keep making the garden nice, and otherwise spend money only on paint – the buyer would not appreciate anything else. Energetically, I do not see you threatened by investment obligations, and no amortization is expected during the further holding period. If your son is to be allowed a second children's room to lounge around with friends, neither the flooring nor the radiator need a building permit (worst case in a "storage room"). Don’t drive yourself crazy and set aside free money well diversified for the dream house. Turning a terraced house with bills into a dream house is economic nonsense. You make it nice with coins, and that’s good. On the real estate ladder, I wouldn’t bind ballast to my feet; another step's decoration won’t give that back to you (and if it does: nowadays there are "films" for that as well).

For a house built in 2007, I would honestly not waste a thought on an energy-efficient renovation or replacing a working gas heating system with a heat pump. Financially, it only pays off, if at all, in the long term. And in 15 years, the heat pump will already be so old that it will have to be replaced when selling. What gas consumption do you have? In the last few years, we had about 16,000 kWh including hot water. Through savings, we are now at about 12,000 kWh. As long as electricity is generated from gas and therefore the electricity price continues to rise, a heat pump simply does not pay off for us. At least not at the current prices for the devices.

If you can make good use of the space in the attic, you should definitely go ahead with the expansion. Whether it is living space or not, it is an added value for the buyer, as long as he does not want to rent out the house.

We currently live in a semi-detached house from 1997 with four people. It is only 105 m², but without the finished attic, which does not count as living space. Now we have about 120 m² including the attic, thus one more room and a layout that suits us better than the old house with 125 m² and four rooms.

If we had known three years ago that we would now have 1,500 EUR more available per month, we could have been a bit bolder. So now we can save more and inflation does not affect us as much. After replacing the stairs to the attic and renovating the bathroom last year, this year either the kitchen and/or the guest toilet are due. The money we did not spend three years ago is now being invested in modernization so that we can live in a way that makes us feel comfortable.
Once the children have left the house, we would also like to move again, as things stand now. But that’s no reason for me to start modernizing the house now for a potential buyer.