Builder of extension asking for more money during build

I'm looking for advice about our builder who we think is trying to shaft us.

I'm in the process of having an extension built to the rear of my property. The builder is two weeks into an eight week job. We had a disagreement because he was asked to re-do some work because of mistakes (not following the tender drawings). Now he is asking us for extra money because the back garden is water logged. The builder says that the water is hindering his progress on the build, and is now looking for one week of additional machinery and labour costs. We haven't been given a figure for the cost yet.

He also tried to swindle us for 2k on the price of our sliding door. We priced it independently with the same supplier after getting suspicious of his high quote.

Our architect worked previously with the builder and recommended him to us. We signed the standard RIAI contract with the builder. Our architect is project managing for us and says that the builder is entitled to ask for costs for unforeseens. Is this correct?

Firstly the builder must follow the tender drawings.He is entitled to extras for the additional time caused by the water logging of your garden.Get your architect to agree a reasonable amount.
The signed contract must be adhered to.

Did you get a QS to scan & verify the materials lists or did the architect as project manager do it?

Clonback & mathepac thanks for your replies.

No we didn't use a QS for the project.