Is a 5-meter floor plan width too narrow?

In England, such narrow houses as [Reihenhäuser] are not uncommon. Take a look at designs from there as well. You’ll often find interesting things.

Best regards
Michael

I actually don’t see a problem with that, since unlike with a townhouse you don’t have to place the front door on the narrow side. This way you don’t have a long, narrow corridor in front of you, but can enter on the wide side and then go left and right into fairly spacious rooms (narrow side 4.20 or even 4.70, that’s quite decent!).

It's not quite that simple. The terraced houses in the UK are 2 or more stories. Additionally, they often have narrow, tube-like rooms. The OP, however, wrote about single-story construction. In this case, the design of the roof and possibly the use of bay windows becomes very interesting.

I see the multiple stories in terraced houses more as a disadvantage. Since you cannot build lengthwise (and the OP has plenty of that available), you have to build upwards. This then leads to tower-like structures, where in the extreme case there is one room per floor. In our family, we have one or two of these "towers" and I find them terrible. I would much rather have a stretched-out design, where you can enter from the side and have a staggered storey with a large roof terrace.