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Seatbelts. Fitting extra. - FotheringtonThomas
What, if any, are the legalities should one fit an extra lap seat belt in the rear of a vehicle?
Seatbelts. Fitting extra. - Dwight Van Driver
None as far as I am aware providing there is a proper seat belt anchorage point on the chassis.

dvd
Seatbelts. Fitting extra. - Statistical outlier
No idea on the legal, but please bear in mind the practical.

A lap belt will inflict pretty horrific injuries on the wearer in all but the slowest of crashes - crushing of internal organs, horrific whiplash or paralysis. It really only serves to protect the occupants in the front seat from a flying passenger; not much more.

I really wouldn't unless you have no choice in the matter.
Seatbelts. Fitting extra. - doctorchris
Check with your insurer that they are happy to insure with this modification. They will, at the least, need to know that the car has changed from a 4 to a 5 seater.
Seatbelts. Fitting extra. - gmac
You don't say if you are planning to use the lap belt or not.
The car has been given type approval based on what the manufacturer submitted.
This is the number of seats entered on the V5C.
You could put ten lap belts in the car but it would not make it a ten seater.