Property values.
The same issue is coming up everywhere. In the old days, it was about mobile home parks. And subsidized housing (which tends to be "as small as the builder can get away with"). Now it is tiny houses. But the basic problem is the same:
Existing homeowners, (and their elected officials)and their elected officials, know that if there are a lot of smaller/cheaper housing units then it will, inevitably, lower the value of the larger units. If you currently have a large mortgage and a change will make your house be worth less, you want to prevent that from happening. Ironically, allowing tiny houses as add-ons to existing property does the exact opposite - having a second unit (as a rental or for a relative) increases property values.
In certain cases - with the smallest/simplest "houses" - there are very real differences that can very seriously justify not allowing them as the only building on a residential property:
- Bathroom - including toilet, sink, tub or shower
- Kitchen - oven + cooktop + sink + refrigerator
- Minimum living space per person - this is a little more subjective, but it is quite clear that while a full-size house might be fine for anywhere from 1 - 8 people (and depending on the size of the house, possibly more), a tiny house might be practical only for between 1 - 3 people.
- HVAC - This depends a lot on climate, but building codes normally enforce certain minimums so that a house can be kept at a comfortable temperature year-round.
So a "super tiny" house that doesn't have its own bathroom and/or kitchen would be, arguably, inappropriate as the only building on a residential property. The Attefallshus appears to satisfy these basic "livability" requirements. Which, to me, indicates the restrictions are based much more on NIMBY than actual practical concerns.
Many other things are more subjective. For example, many apartment buildings are not designed with laundry machines in every unit, so requiring a washer/dryer in every tiny house would seem a bit arbitrary.
Long-term, it seems to be generally better in many ways to allow for a variety of housing options. But in the short-term it is always "how does this affect me". The people living in the big houses count. The people who would be living in the small houses don't count, because they are not there yet!