There are seed spawners just next the 15 Summercourt Hill lot. Their radius overlap into the lot, so that, sometime, a seed is spawned on the lot. When this happen, the empty icon is changed to empty house. As soon as you have 1 thing (other than the trashcan and the mailbox) on an empty residential lot, the game considers it as "built". You can always bulldoze an "empty house lot" to remove anything that has spawned on it.
I have trouble sometimes to move new CAS sims into emtpy lot. I find the user interface is not 100% clear on that. What works great normally is to hit: Change Active Household (big house round button in Edit Town) => Move in Sim (and you pick up your Sims in the household bin). This way, you can choose empty lots too.