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Are the following sentences well structured?

We are expecting our engineer on site in the morning to investigate the pipe noise in the building.

We require access to your flat so that he can run your water outlets to determine the possible cause.

Are you happy for us to use your spare key from the concierge's office?

We look forward to receiving your response.

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Yes, they are all OK. (I wouldn't start a new paragraph for each sentence, but perhaps that is not your intention in the final text.)

You could consider stating or confirming the day/date of "the morning".

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GPY has already mentioned that your sentences are good. I will rephrase your sentences slightly so that they seem more natural to my ear.

Goldenboy1We are expecting our engineer on site in the morning to investigate the pipe noise in the building.

We are expecting our engineer on site in the morning to investigate the noisy pipe in the building.

My sentence isn't any better or worse than your original. In fact, some might argue that the original is better because it might not be a pipe. The noise might just sound like it's coming from a pipe.

Goldenboy1We require access to your flat so that he can run your water outlets to determine the possible cause.

We require access to your flat so that he can run your water taps to determine the possible cause.

In my neck of the woods, water taps is more common than water outlets. If you check Google Ngram, you'll see that term water taps is more commonly used. Read the comments below. GPY makes a great comment. In effect, he says that outlets include toilets and appliances, so outlets is likely a better word.

Goldenboy1Are you happy for us to use your spare key from the concierge's office?

Are you okay with us using your spare key from the concierge's office.

This sounds more natural to my ear.

Overall, well done.

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Comments  
MountainHikerIn my neck of the woods, water taps is more common than water outlets. If you check Google Ngram, you'll see that term water taps is more commonly used.

The only thing I would say about this is that "water outlets" is more general than "taps" and could include e.g. toilet flushes or appliance outlets.

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GPY, good catch!