With typical "string inverters" adding solar panels is possible but it can be complicated. Inverters and solar panels are designed to "fit together" in manufactured approved configurations. Because there's an ideal number of solar panels that should be configured up to a specific inverter, a whole mini system - solar panels and inverter - would most likely need to be added if you want to increase the size of your system. In many cases it's better to oversize your system a bit if you think you'll need more electricity in the next few years.
Choosing an Expandable Enphase Micro-inverter system will alllow you start out small and expand as you are able. You can literally add one panel and one micro-inverter at a time.