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Flatten Slope with Waste water & storm water manholes

The yellow encircled line (see attached photo) in my property is a bit of a steep slope. To utilise the land for more backyard space, Now I’m thinking if it’s even possible or allowed to flatten it and have a retaining wall? Main issue is that two public manholes for stormwater & waste water are on that slope, can the coverings be lifted up by adding lifters and lids set to the flattened level. Also, slope is on a 1 in 100 year flood plain. Please contact me if you can help.