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Materials have actually fallen slightly recently, labour is definitely the issue. The cost of living is obviously the main driver but there are still people taking the piss. The architects we're using find contractors quote more for the same job if it's a bigger house in a nicer area.
I'm sure you can do what you're saying you can for those figures but most people in London will have to hunt around to get those prices and I think you'd be talking very basic.
The cost of the materials has not generally gone up much from a year ago. The cost of labour I'm not so sure about. I know you can still get a basic d ormer for 40k. My polish builder is quoting 55k for the extension I had done for 45k a year ago.
Unless both extensions are enormous and very high spec 110k should be a reasonable budget. But all depends on the spec really and how much the builder is marking up the materials.