I had a rough quote recently, all in it was near 100 but the actual extension part came to 25k. Glass/doors can range from 1000 to 25000 or more. Same with kitchens. My pals renovated recently and they budgeted 65k and I think it came in at 130k in the end. It’s a terrifying can of worms.
That’s less awful than I imagined, but yeah I can see that ‘it depends’.