Do I have to buy a new car seat

[4theanimals]

my 5 year old has outgrown the car seat where there are two straps that come over the shoulders and fasten at the lap. She still easily fits in it, but the straps aren’t long enough to pull around her anymore. Instead of replacing it as it’s still in vgc, can I just use the car seatbelt to pull over her?

Read the instruction manual for the car seat!!
Can it be used without the harness?
Is your child the correct height and weight for the seat?

No!!!
Seats that are designed to be used with the seat belts have guides for the seat belt and hold the seat belt at the shoulder and the low cut sides guide lap part so it holds the child.

Trying to put the seat belt round a carseat that's not designed to means that the child could slip under it. Or come flying out of the seat

Without knowing what seat it is, what height and weight your child is, and therefore whether it’s legal and safe to do so, we can’t tell you.
You’ll need to use your car seat manual to see what’s allowed and at what height and weight your car seat is outgrown, or needs altering to a different setting.
But using it in a way it isn’t intended to be used will be extremely dangerous in the event of a crash.

Most 5 year olds should be in a high backed booster using the car's seat belt, as their height and weight will exceed that of a harnessed car seat.

Some harnessed car seats convert to a high back booster (Joie do one) but most don't so if yours doesn't you'll need to buy a new seat.

Good news though, high back boosters are cheap and really available 👍

It really depends what seat it is - but most seats of the stage you describe do up to either 18kg or 105cm, which most 5 year olds will be well over.

If it's one which converts to high back booster, then you usually need to do something to convert it involving tucking away or un-threading the straps and unhooking the strap mechanism from the headrest, then it slides up.

These seats have guides on the headrest and next to the child's hips to route the adult seatbelt through.

If you don't have those belt guides on your seat - they will be obvious and highlighted in red or green - then you can't use it with a seatbelt, it wouldn't be legal or safe.

It is most likely you need to buy a new seat. But if you can come back with the name and model or a photograph of the seat, we can give more info.