Is the floor space index adjustable against an additional payment?

Lately I've been watching a lot of HGTV. The tiny houses they always build in that one show are all under 30m² or even 20m². And what they manage to fit inside is really cool. I don't know what purpose your additional little house is supposed to serve, but another option would be not to build it as a permanent structure but mobile. If it is not constantly inhabited and not firmly anchored to the ground, as far as I know, the floor area ratio doesn't matter.

Floor area ratio 0.209

...doesn't sound to me like 0.210 would be desired - and even more certainly not. How does this odd number come about - was the plan made when the area was already largely developed and then derived from the actual situation?

Where is there such an irregular floor area ratio?

Nowhere, it was only calculated as missing. I would have the preliminary building inquiry done by an architect. He knows the building authority and the rules... and the ways.

Nowhere, it is only calculated as missing.

I clearly understand the OP to mean that this is really the floor area ratio specified by the development plan, but I am also surprised by such an odd value. With 0.271828182845904523536 I would have suspected a plan preparer with a mathematical background, but like this? No idea. OP, please say something about it. :)

Thanks to everyone, here are the details: 1. Berlin Neukölln - long developed single-family residential area Floor space index 0.209 (information and plan from the building authority) 2. Should be constantly inhabited (son // new property too expensive - prices Berlin!!) therefore Tiny & Co too small, aside from that Tiny houses also need to be approved in Berlin. Maybe it’s more generous deep in the heath. 3. The floor space index increase calculated by me takes into account a house with a 60m² floor area, which still seems acceptable for permanent residence. 4. The tip from ypg (architect) does not seem promising to me due to experiences made within the circle of acquaintances and because of costs, since if construction is not possible the project is dead. This is not meant as a criticism of architects, but I could fill several pages with experiences that cause me slight doubts about the training of the professional group "A". (I do not mean drawing, calculating etc. — but the practical applications and dealing with authorities etc...) ...therefore the old saying still applies to me: "What you can do yourself..." 5. To be a bit more positive: in Berlin it is possible (once a week from 10 am to 1 pm) to get free advice and plan inspection at the building authority. Unfortunately, this opportunity has been canceled for about a year due to Corona. (Why? We could keep distance and use plexiglass. Well, another topic...) That a possibility of exceeding the development limits (not meaning the 50% secondary areas) is possible, I have already determined through aerial photographs of my area available to me, the question is only up to what percentage this is possible. I do not want to submit building applications etc. and then experience everything being rejected. I have also found out that a written, fee-based preliminary building inquiry is possible, which unfortunately is also currently not processed promptly due to "C". Apparently, the building authorities (in B) have always been understaffed, which is now particularly noticeable during Corona times. I will research the mentioned inquiry more thoroughly again.

In many residential areas in Berlin, a site coverage ratio of 0.20 applies, slight overbuilding is tolerated but 0.21 must not be reached. In our case, 0.21 + 4m2 was approved by my wife in a pleasant conversation at the office, the architect’s justifications were given to them, in our case, floor area ratio not fully utilized, 42cm exterior wall, eco-friendly building materials, WP