Concrete Lintel Load Calculations Needed

I’m putting up a 3.8 m × 3.8 m single-storey rear extension. A 110 mm sewer runs beneath each side wall; local regulations insist on 150 mm clearance all round, so the wall will bridge the pipe with pre-stressed concrete lintels spanning roughly 410 mm between ring-beam piles... (Budget: £20 - £250 GBP, Jobs: AutoCAD, Building Architecture, Building Regulations, Civil Engineering, Construction Engineering, Construction Management, Structural Engineering, Technical Writing)