JenDre

No news.
But "Roughen" is probably my #1 Illustrator feature that I miss in Affinity.
There are several workarounds/workflows that can simulate a "roughen" effect though. I've posted quite a few in the past years:
https://forum.affinity.serif.com/index.php?/search/&q=roughen&quick=1&author=loukash&search_and_or=and&sortby=relevancy

Better late than never: Thank you all.
Your input helped me to understand the "operation method" of AF Designer (1 + 2) and my individual needs when working with sketchy lines.
@NotMyFault Thanks to your method, I am able to create the desired look. Even if it's more time consuming and with less individualisation options than the original function in Illustrator. Thank you!

To generalise the principle. If you need a rough line, draw a rough line with the pencil tool and basic round brush. Add invert adjustment. Select a suitable rectangle covering the rough line, export selection as PNG, and create a new intensity brush from PNG. Use this brush on any curve (stroke).
 

I simply used the screenshot of the OP to create a new intensity brush.
works like a charm.

Dear all,
for architecture presentations and documents it is common to use sketches or sketchy looking drawings. You can find bunches of examples by searching for "architecture hand sketches" in the web.
So far I used Illustrator to produce this type of drawings. To create this specific look you can transform straight lines into sketchy lines just by a simple effect (Effect > Distort & Transform > Roughen). Attached you can find an example. This effect works not only for lines but for vector objects in general. The good thing is that this effect doesn't change the object nodes; no additional nodes will be added. This makes it comfortable to use.
Unfortunately, I did not find a similar effect in AF Designer. Even the web didn't give me any tipp how to do so. Instead of that I just found a topic from another guy asking for sketchy lines in general, but his problem seems also not to be solved yet: forum.affinity.serif.com > Topic 146665 > sketchy-lines-in-af-designer/
Is there an similar effect in AF Designer, which I have not found? If not, what is the chance that a solution will be developed in the near future?
Thank you in advanced.

Unfortunately no. Affinity follows the golden rules of diplomacy and never comments about future plans. The average time between a feature request and implementation varies between 3 month and more than 5 years with the majority on the upper bound.  

Guess where this one lies!