john cunningham
You are indeed lucky that your chamber ( manhole ) is at the beginning of the line. You can alter and adapt this chamber in height to accommodate your new floor level. Then fit a new cover which must be a double seal and you fill the seal with something like vasolene to stop the gasses and smells that accumulate in sewers coming into your house. There are various covers available, some are open grids that you fill with concrete or screed to finish level with your new floor. Some are made to take quarry tiles or whatever you wish to finish your floor with. Your architect I'll show this on his drawing and the local authority will inspect it in due coarse. JC
