Considering buying a house that has an extension on the back classed as utility and porch type area.

I assume this has come up in a survey, ask the surveyor his opinion as he has seen the damage first hand. He will have an idea on a good route forward. But in my own view (easy for me as i can do the work myself) i would replace the lot in one go and do it right. Without seeing the site your pipework shouldnt be a major issue to repair unless it runs under the extension, its usually a case of simply digging out the old pipe amd fitting new assuming there isnt an underlying reason why the pipes have moved other than the extension pushing them out... You will note my use of the word 'assume' quite a bit, im just guiding you without seeing the issues first hand i cant be any more accurate. Good luck Dan

I would remove the porch.If it has insufficient foundations then you can bet your life the entire thing has been a half arsed job.Also if this construction has damaged drain pipes,then what would be the point in repairing the drains if the reason they are damaged,remains.As pointed out by dan,speak to the surveyor for advice as he is in a much better position to advise.If you buy the house deduct the cost of removing the porch and repairing the drains from the asking price.

i would consider buying something else