Additional Cost of building a rear extension over a public sewer

I am purchasing a property in Perry Barr, Birmingham area and want to build a Kitchen/Diner extension after purchase to the rear of the property spanning the entire width of the house. But the Water & Drainage Search Repot highlighted a Public Foul Sewer of 150mm within 3 meter of rear wall of the property. I phone the Seven Trent Water Company and they confirmed it and have said that I would need a 'Build Over Agreement' and I need to attach the CCV footage or change the sewer pipe. I have received quote for the cost of pre and post work CCTV footage , but I was wondering if the changing pipe would be better solution in longer term. Also I assume I need to dig deeper than 1 mt for the extension where the pipe goes provide a structure for it. Is there any body who can advice me the cost for this additional structure and replacement cost of the pipe incl. labour. A rough estimate will do. The length of the pipe from one fence to another is roughly 20 ft, and the diameter is between 150-200 mm.