Car Seat to Booster Seat: When and How to Make the Switch in the UK

Switching from a car seat to booster seat, a transitional child safety device used after a five-point harness seat and before a regular seat belt. Also known as high-back booster, it's designed to lift your child so the car’s seat belt fits properly across their body. This isn’t just about outgrowing a seat—it’s about keeping your child safe during every ride. In the UK, the law says kids must use a child car seat until they’re 12 years old or 135cm tall, whichever comes first. But many parents rush the switch too early, thinking a booster seat means more freedom. The truth? Staying in a five-point harness car seat longer is often the safer choice.

Before you move to a booster seat, a seat that positions a child so adult seat belts fit correctly across the hips and shoulder, ask yourself: Is your child tall enough? Heavy enough? And most importantly, are they mature enough to sit still? A booster seat doesn’t hold your child in place—it just raises them. If your 5-year-old wiggles, slouches, or takes off the seat belt, they’re not ready. The child car safety, the practice of securing children in vehicles using approved restraints to reduce injury risk experts agree: keep your child in a harness seat until they hit the maximum weight or height limit, even if they’re over 4. Most UK car seats allow up to 18kg or 105cm. That often means staying in a harness until age 6 or even 7.

And don’t forget the UK car seat laws, the legal requirements for child restraints in vehicles across the United Kingdom. Any booster seat you buy must be EU-approved (ECE R44/04 or R129/i-Size). No second-hand seats without a clear history. No makeshift cushions. And never, ever use a booster seat without a lap-and-shoulder belt. A lap-only belt is dangerous for kids. The right booster seat should have a high back with side impact protection. It should also be easy for your child to get in and out of—because if it’s a struggle, they’ll find a way to bypass it.

Some parents think moving to a booster seat means they’re doing something grown-up. But safety isn’t about milestones—it’s about fit. If your child’s shoulders are above the top harness slots, or they weigh more than the seat allows, then it’s time. But if they’re still small, still wiggly, still falling asleep in the car? Keep the harness. You’re not behind schedule—you’re being smart. The posts below give you real, no-nonsense advice from UK parents who’ve been there: when to switch, what seats to avoid, how to check if your booster fits right, and why that extra year in a harness could make all the difference.