Whether you can add panels depends on the config really. For best performance the inverter is usually undersized for the array, so if your inverter is already undersized and you go sticking more panels on it would be limited at the converter anyway, unless you get a new inverter (now well north of £800 I imagine plus fitting)
I'm limited by my Feed In Tariff agreement to what is on my roof at install time, even down to the exact type and output of panel - I'm not sure what I'd have to do if one broke, but that's fairly rare. I don't believe this applies to new installs as there is no/little FIT. I think if I extended mine it would have to be a separate array and inverter but I'm not sure - and I have no more roof space anyway!
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