Quote taken from light vehicle modifications:

Seats
Owners should ensure that the seat and its anchorages comply with any applicable ADR requirements (possibly by certification from the seat manufacturer or by an engineering signatory) and the appropriate seat belts are fitted to all seating positions.

Vehicle Standards Bulletin No. 5 issued by DOTARS covers requirements for the manufacture of additional seats.

Seat belts
The level of safety provided by seat belts must not be reduced. Seat belts must not be replaced by belts of a lesser standard or with second-hand belts. For example, lap only belts must not be used in place of retracting lap/sash seat belts. The use of a retractable seat belt disabling device is not acceptable.

Seat belt anchorage points have been carefully chosen and tested by the vehicle manufacturer. Do not alter the location or strength of seat belt anchorage points without advice from an engineering signatory.