We currently have 4 solar panels on our roof, existing I believe from original construction in 2013, we moved in in 2017; and I think there is sufficient space to add additional panels.

For those that have installed or expanded a residential solar system, what would I need to know to consider this?

Cost per panel?
Upgrade sub-panel(s)/inverter/etc?
Install costs?

Hopefully not huge photo of the house from Google maps, with estimated placement of expansion(s).

How about a ground rack on the far side of the pool, or in the side yard to the left of the house?

You need a new planet

Left side of the house in that photo is now occupied by a kids playground structure. Google maps is a couple years behind on our neighborhood.

My assumption, with no prior knowledge, was that it would maybe be easier to expand if it tied into the existing system?

Do you heat the pool?

If so, solar heating water is much more efficient than solar panels are at creating electricity.

My understanding is the preferred direction for optimum solar collection is facing South so whichever roof panels face that direction.

1) Need to know orientation of your house--South facing roof? Assume pool faces south.
2)2013 panels are obsolete-scrap them. Redo the entire roof facing the pool with new panels. Add an additional row -landscape orientation. Add panels to other roof to the right-maybe 6 in portrait orientation. Add 10 + panels to the other roof in portrait orientation. Another 8 panels on the next roof, another 6 panels on next roof.
Now you are up to 36 panels-better effiency, more electric production for that size house.
I run an 18 kWh/yr system. LG 360 with Power Optimiser. E mail if questions.