Water connection outside from the kitchen - possible?

During pipe installation, installers must pay attention to the purpose-fulfilling flow rate when choosing the pipe cross-section. That means normally connecting each "standard" supply line or fitting DN15 in parallel to the distribution, riser, or floor pipe. Although it is possible to connect an additional consumer to the outlets, this is not compliant with regulations and the consumers influence each other when used simultaneously. To avoid stagnation (germ formation, "dead water"), the branch should be as short as possible and/or not be permanently closed.

When installing pipes, the installers must pay attention to the purpose-fulfilling flow rate when choosing the pipe cross-section. That is, normally connect each "standard" supply line or fitting with DN15 in parallel to the distribution, riser, or floor line.
Connecting an additional consumer to the withdrawal points is possible, but it is not according to regulations, and when used simultaneously, the consumers influence each other.
To avoid stagnation (germ formation, "dead water"), the branch should be as short as possible and/or not be permanently closed.

That means it could be that I cannot use the garden and the dishwasher at the same time :-(

That means it might be that I couldn’t use the garden and the dishwasher at the same time :-(

In my opinion, a dishwasher is probably not the problem; it consumes about 7 liters of water per cycle if it is a current model. However, if the lawn is to be watered outside, opening the tap at the sink could cause a drop in pressure if the pipe diameter is not large enough. I am not a plumber, but for outdoor watering, I could imagine that a larger diameter would be more sensible than for a kitchen. Otherwise, the sprinkler turns into a trickle. My brother has an automatic lawn watering system (about 300 m² lawn area) and it already operates in 2 sections so that the range is not too limited.

That means it could be that I can't use the garden and the dishwasher at the same time :-(

If the same person is responsible for cooking, clearing up, and watering flowers outside, that shouldn't be a problem. No, seriously: if you know about the emergency solution and can adapt to it (e.g. not using the garden connection as a constant consumer while cooking), you can come to terms with it. By the way, depending on the dishwasher and the type of heating system, it is possible to connect the dishwasher directly to hot water (but like many things, this also has both advantages and disadvantages).

but for an outdoor irrigation system, I could imagine that a larger cross-section would be more sensible than for a kitchen.

To my knowledge, only the WC flush valves should have larger cross-sections, e.g. DN20, everything else DN15.

Our shower also has DN20