I'm pretty sure that houses and gardens both require civ techs, and why the hell do you want a level 3 city that can only build huts?
You can build huts and
farms gardens with no civics techs or doing any research in any line whatsoever.
Plant a city, build a mill (costs 10 gold, provides 2 materials) and a workshop (costs 10 gold, provides 1 materials), both can be immediately built. When you get 2 materials build a garden. The garden provides 2 food which allows a hut.
The value of refugee camps is that for only 5 materials and without building anything else -- no farm, no huts, no nuthin -- you can get to 90 pop as the pic shows:

[for some reason that game started me with 25 materials, not sure why, if no materials at start then put in the mill and workshop for the materials]
90 pop is city rank 2, and almost rank 3 (100 pop). Add the garden and hut and you're at city rank 3.
The question isn't "why the hell do you want a level 3 city that can only build huts?" but "what can you do with these people that are free for only 5 materials and waiting for them to arrive?"
Note that the refugee camp description says it adds 40 pop, but in effect it adds 65 -- without it the city will reach 25 pop 'sans huts', with it it'll reach 90. The 'extra' 25 pop is because the camp gets the city to rank 2 and pop then increases to 50 'sans huts', accounting for the 'extra' 25 pop. This increases the value of camps.