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Estimate for stormwater drain extension

I have an existing stormwater drain which extends down one side of my house to the back. At the end is a downpipe. Going back up the other side of the house there are no downpipes, and about 17-20 metres up that side is a gulley on the roof which is causing overflow issues. I want an idea of the cost of one or possibly two additional downpipe inlets being added (there are two gulleys on that side and the other has to be passed to reach the problem one anyway). There's an earth / gravel area directly beside the existing downpipe, then about 4 metres of concrete slab, an earth/garden area, then a 45cm strip of garden along the house edge where the blackwater connections, etc are. The extension would have to go under the slab as far as the garden, but could potentially (I think) run above ground from there.