Under 10 kW is unproblematic.
Meter system open or retrofit, put in another meter — done.
From 10 kW onwards, an additional dedicated meter slot as a 100% meter only for the PV becomes necessary. So probably a new meter cabinet.
From 30 kW — small transformer system. Even more space needed.
but this should never ever lead anyone who could mount a solid 25 kWp on the roof to stay under 10 kWp, please please don’t. B and W got a good 21 kWp on my brother-in-law’s roof affordably including a new or additional meter cabinet. Unfortunately, I couldn’t get along with my contact at B&W. Three times he tried to sell us something that was neither sensible nor what we wanted (and had communicated). So it works, but it can be difficult.
I also thought for a long time that under €1000 per kWp should be possible. But for small systems (6–9 kWp), unfortunately that is by far not the case. The market situation right now is rather bad for buyers. For 25 kWp, however, I could imagine it fitting if no major extra work arises or complications occur.
We are currently getting 365 Wp modules from JASOlar. They cost €119 net each. The whole system (mine) with about 6.5 kWp, 4 kWh battery, 2x controllable wallboxes (key, app-capable etc.) including complete installation etc. around €17k including VAT... unfortunately so expensive. But subsidies still apply (about €2.5k).