Planning single-family house with bicycle workshop

Why? If the room meets the same requirements as a living space, it is just as expensive. And for a workshop in winter, you have the same requirements or what do you want to leave out?

Of course, the room has to be built completely differently and, for example, as a wooden extension to the carport, it costs only a fraction of the living space in the house. We're talking about costs for the workshop in the low thousand-euro range. Dry, airtight, electricity, electric heating (fan, IR or whatever). The workshop doesn’t have to be insulated and solidly built. You just have to be able to work - and when you do, you turn the heating on. A small garden shed would also work. I would never come up with the idea to sacrifice precious living space for regular work on bicycles.

Moreover, a workshop outside the thermal envelope must first be laboriously heated up, which can sometimes take hours.

It takes less than minutes with a proper fan heater. You don’t need 22 degrees, but a temperature at which you can work. And then it’s only 15-20 cubic meters.

The workshop does not have to be insulated and solidly built. You just have to be able to tinker - and when you do that, you turn on the heater.

In this case, I understood it differently. The OP works frequently and a lot in the workshop, especially in winter.

It takes less than a few minutes with a proper fan heater. You don’t need 22 degrees, but a temperature at which you can work. And then it’s only 15-20 cubic meters.

Heating up an uninsulated room in the deep winter with a fan heater takes forever and you’re still cold. We have an insulated workshop without heating. It takes hours with a fan heater and an additional radiator. We are considering installing an electric heater. But heating up still takes a long time, by which point you’re usually done before the room is warm. So here the OP should carefully consider when and how often during the week he needs the room. Practically, a room that could be used for other purposes in summer would probably be best...

You are still young. Are children planned?

In this case, I understood it differently. The original poster often works a lot in the workshop, especially in winter.

Unfortunately, the cold, wet season is the one in which the bike needs the most maintenance, or at least the everyday bikes that are also ridden to work.

As I understand it, it’s simply about maintenance for preserving value as well as possibly a hobby cyclist who occasionally wants to replace something on their bike. And if it’s a nice standard garage, possibly with a daylight window, then you can also work nicely in there if the room is brought up to 15 degrees in 10 minutes. No one needs constant 20-22 degrees with underfloor heating, which is rarely found even in a single-family house.

... if the room is heated to 15 degrees in 10 min.

Never! Honestly, we have gained experience with this. You won't heat a workshop to 15 degrees in 10 minutes if it's 0 degrees outside. And it shouldn't be as small as a phone booth either if you want to get in and out with presumably at least 2 bikes. In addition, the OP requested a connection to the shower. I fear you are too much guided by your own ideas of cycling. To me, this sounds like 2 sports enthusiasts who spend most of their free time on their bikes.