Is construction with prefabricated walls possible? How and who does this?

Hello everyone,

A few years ago, I took over my grandparents' house. What I never liked about it was that there is no proper bathroom solution. The main bathroom is in the basement and is difficult to heat (house is from 1954).
On the ground floor, there is a guest toilet, which I am now planning to expand. A large terrace borders the guest toilet on the outside wall, and an extension of 3 x 4 meters would fit well there.
My idea was to make the necessary walls from prefabricated elements. Which brings me to the question: are there companies that offer a custom-made solution for this? Or does the forum have other suggestions? What do I need to consider?

Best regards

an extension of 3 x 4 meters

... you will hardly find a turnkey company for that, actually there is only one solution for you, find a local architect who will design your bathroom and then look for a company that will build the bathroom for you ... and what construction costs have you budgeted for that?