You can usually add a battery later, but the decision you make now about the inverter is what determines how easy and cheap that will be. If you fit a hybrid (battery-ready) inverter at install time, adding storage down the line is a relatively straightforward job. If you go with a cheaper string inverter to save money up front, you'll likely need to replace the inverter when you add a battery, which eats into the saving.
There's also a VAT angle that catches people out. A battery is zero-rated for VAT when it's supplied and installed with the panels under the same contract. Add one later and it isn't: it's 13.5% or 23% VAT depending on how much of the price is goods, and usually 23% for a battery on its own. So if you're fairly sure you'll want storage, buying it with the panels saves you the VAT, and you don't need to qualify for any grant to get the 0% rate.
Another approach is to go battery-ready but skip the battery at first, watch your real generation and consumption for a season, then size the battery to what you actually see. It's a low-regret path if you budget for the hybrid inverter premium (often around €1k extra) and accept the VAT on the later battery. A €600 battery grant is expected in Budget 2027 on 6 October 2026, but it isn't confirmed, and it isn't known yet whether it will cover a battery added to an existing system.