Hello parsm, I would not think so if all is properly insulated and there is heating, you could consider underfloor heating. Other ideas would be a polished concrete screed flooring.. or large stone even a rubber flooring like Dalsouple could be fantastic... Who is putting the extension in... They may have some advice as to what is recommended, but I can't see really why it would be a problem if done really well and floor is properly laid and insulated as well.. : ))

This comment is a bit late, but we bought a house last year that was extended in 1992 over not one, but two, manhole covers (kitchen and cloakroom). Both had been tiled over but with sealed hatches that could be lifted in case of emergency. In 25 years this was never necessary but the idea of leaving them in tact was a little unpleasant. It's a public sewer that includes several neighbouring houses. We just undertook an extensive house refurb and moved the manhole access points outdoors and permanently sealed the internal ones. It wasn't as complicated as we originally thought. What you want to do as a starting point is get a drain surveyor to come and inspect the routes and flow of waste. We used Power Rod (we are based in Hertfordshire) and they came with their camera on a hose reel and mapped out our plumbing routes all the way to the mains in every direction. Then your builder will be able to take it from there, or you can hire a specialist grounds company to sort it out. If it's any help, in your head you may have this idea that raw sewage is just gushing under your floorboards but in reality, when I looked down the hatches, it's really just a matter of a few pipes several meters down. It was surprisingly more sanitary than I expected and I found that reassuring.

150k is a generous budget but a 2 storey could absorb that! Loft extension about 40k minimum. A new bathroom 7-10k. New kitchen 10/15k. Structural work 10k. Redecoration 5K. New siding doors 5K. None of these are high spec prices. So you need to prioritise your needs based on your family requirements. Why do you need a downstairs loo and a separate full bathroom? Better to have a utility, loo under stairs IMO. Bathroom upstairs is a must. Do you need the third bedroom - if that is a must then the loft extension is the most cost effective way of doing this without the need to extend. Stair placement is the key consideration but your stairs may give you an option to construct this without the need to sacrifice precious space upstairs. I would merge the kitchen and loo area into an open kitchen/diner.

These are all great thanks so much, lots of food for thought, I think it's hard to imagine your own house any other way! It's not currently a drive, no. We have 2 parking spaces to the front of this area. Currently there's a side gate and pathway next to the house, then a very large shed that needs pulling down, with a patio and small lawn behind that. Not really used it doesn't catch the sun much. So really dead space. I like the idea of the two storey smaller extension as could open up the front of the space to be an extra space for a car as well. I need to get outside with the tape measure as I'd definitely prefer to keep a gate and pathway down the side to access the back garden. Next to that is a wire fence and then school grounds so no close neighbours to annoy that side.