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where are you based and what type of property do you have out of interest? Personally neither giving you any idea of what they charge isn’t a good sign it should be an easy package for them but then have either of them gone through a brief of what you would like?
It’s hard to reduce the cost of extensions sometimes as there are basics that cannot be avoided and as for systems which require less foundations it’s building control that will determine these anyway.
It’s always a good idea to look in your council planning portal and see who recently had an extension done and just knock on their door and ask if they wouldn’t mind you asking them about it! People are usually more forthcoming and it’s so beneficial to visually see and talk to other neighbours too?