@Cherryblossom200 I categorically am not an expert but I'll share what I've found out so far.
We have bought a traditional brick built house - we need to extend to create an annexe and make the main house bigger (making an annexe for elderly DM). I had concerns over costs - everyone told me to budget for at least 15% more as it never costs what is quoted - and also the length of time. My DM is disabled and I have 2 disabled DC who don't deal well with change/noise/people.
I hadn't heard of modular builds (which are sometimes also described as prefabs but not the same type as was found in the 1950s etc). We used an architect and a structural engineer to draw up plans and technical specs, and then submitted everything for planning permission. This is where we're at now, awaiting the outcome.
I had found a builder, or so I thought, but he started acting really weird and took 3 weeks to come back to me with an answer to a few simple questions about his quote. I'm honestly at my limit with people dicking me around so I thought screw this, and started to look for another builder. And that's when I discovered modular builds.
You can have a whole house as a modular build, or just an extension. There are different types of modular builds - the one we're going for is made with a galvanised steel frame, and steel foundations. Soundproofing and thermal insulation are both excellent. We're having a kitchen in part of our extension - the walls are strong, and you can mount whatever you need on the wall, just like a regular build. You can have bifold and roof lanterns, and all the things you might normally want in an extension.
The company comes out to do a survey first and they'll liaise with buildings regs inspector. Planning permission doesn't really make a difference in so much as whether you do a traditional build or a modular build, it doesn't make a difference to planning. It's the buildings regs guy that makes a difference and he doesn't normally get involved until after you've got planning permission.
Once they've done their survey and got approval from buildings regs, they make the panels in their factory and this is then shipped to your home. This is why it's quicker to install - the whole extension should take no longer than 2-4 weeks once they're on site and putting it together. The outside can be rendered how you want - we will probably go for brick slip so it matches the main house and looks like a traditional build.
The price includes VAT, electrics, paint, floorings, plugs, finishes etc and you can configure what you want and where.
Obviously this is a non-standard construction which will make some people nervous. But it's not like the older types of non-standard construction - these should be fully mortgageable because they come with the same building warranties etc that a brick built extension does. Overseas in Europe and the US, this type of construction is really really common, it's just here in the UK we're a bit slow on the uptake!
Cost is fixed so there's no need to have a big "just in case" budget set to one side as you normally would with a build. Apparently it can go up in pretty much all weathers too, so if it's a winter build there's no delays.
Ummm......trying to think what else I've learnt that might be useful!! I've spoken to quite a few people who have one of these, and I've asked the guy at the company an absolute TON of difficult questions and he's been brilliant at explaining it all to someone like me who knows naff all. I think they estimate it to be 20-25% cheaper than a traditional build.
All of this comes with the caveat that I've not had mine built just yet but it's definitely the way we're going to proceed. Such a short period of disruption is infinitely preferable for my mum and DC, and the cheaper cost is very helpful too!!
If there's anything else you want to know, just ask - I have BOMBARDED the poor bloke with questions so there's a good chance I might have the answer!