[savoche]

What reeftool said. I have a 35 and a 15. I think there's room for the 21 in between. And I want the 70. Whether I find space for a 40 Ltd as well...

The 15 is wonderful, but very wide (obviously). I find it needs a dominant foreground to make interesting pictures. I believe the 21 is more versatile. Or that could be the LBA talking

[Frogfish]

When considering which lens is suitable for your style of shooting then the difference between lenses in their FoV is a critical factor. On a Pentax APS-C the table below gives you an idea of how big a difference exists between your 28mm, the 21mm and the 15mm.

Relative FoV (horizontal) on x1.5 multiplier crop sensor :

8mm = 112.6 degrees
10mm = 100.4 degrees
12mm = 90 degrees
15mm = 77.3 degrees
16mm = 73.7 degrees
17mm = 70.4 degrees
20mm = 61.9 degrees
21mm = 59.5 degrees
24mm = 53.1 degrees
28mm = 46.4 degrees
31mm = 42.3 degrees

So 21mm up to 28mm is a 13.1 degrees or 22% difference in the horizontal FoV - that's huge. If later you switch from the 21mm down to the 15mm that is a 17.8 degrees difference or a 30% gain on your 21mm, again huge !

If it were me I'd buy the 21mm (well actually I'd buy the Voigtlander - it has less distortion), there is a big difference to your 28mm and whenever you 'zoom with your feet' you also change perspective so it's not only not always feasible but sometimes also not desirable, and as you say it is a very good price for that lens in your country, so if later you decide to buy the 15 Ltd. you are not going to lose by selling it on. I have a 15mm and 28mm and TBH I really feel like I need the 21 Ltd or a nice Voigtlander Skopar 20mm/3.5 pancake to fill in the gap