New housing spaces

What is the trick to being able to make use of all these spaces. I was able to get 3 small river houses in a space that takes 6 but the remaining 3 spaces are no longer available to place houses now. Mine is the top - the bottom is someone else's so I know 6 should fit there (although even there it appears spaces where I have my houses don't exist for them).

https://imgur.com/a/t0g4NfL

Also, why does there need to be a trick. Those spaces should accept the 1x1 houses in any order.