I don't understand the obsession with prefab homes. Constructing homes is a well-understood, mostly solved problem. The cost of housing is not high because homes are difficult or expensive to build, it's because the land underneath the house in desirable areas is expensive, and various regulations make it difficult to build high density housing. No amount of streamlining the construction process is going to change that.
We're just going now through the process of building a new house. Land is around 10% of the overall budget (and this is a really sought after area in CA).
We did a lot of research and we were initially thinking of prefab for our home, but we finally decided to go for full custom design. In the end, prefab can really make some costs more predictable, at the cost of some flexibility in the design. Cost-wise, it was almost the same (for our case) but construction would've been faster and more probably more controlled in terms of budget.
My experience is 40%+ in high demand areas. Even in a low demand area, land cost is expected to be 20%+, and this was pre Covid. I imagine it’s much higher now.
You'd have to design it in, but all your components would be preplanned or standardised. You'd know before hand where all the piping and cabling went. Maybe the walls could even be finished in the factory and you just have to attach them together. You could even have standard ways to connect built in storage and kitchens.
In custom houses they actually can and often do pre-plan those things. In practice though plumbers know what they are doing without needing to look at the prints and it isn't important enough to make the plumbers put the pipes where the plan puts them. The architect knows this and rarely puts enough effort into those parts of the plan to get them perfect, just enough to be sure that where space is tight everything can fit.
No plan is perfect the first time. For pre-fab you do a few prototypes and find out and fix what doesn't work, then make the jigs so everything works. For custom work the trademen are smart enough to get it working without extensive pre-planning.
The above only applies to small scale single family houses. As you get into larger buildings: correct planning becomes very important. Eventually things must planed correctly in the design phase or the whole will not work.
If you are willing to pay for it you can get a custom home site built in a week. It typically isn't done this way because you pay experienced people to stand around waiting for things to be ready for them to work. It is much more cost effective to schedule the plumbers for one day and the heating guys the next, then the electricians, even though each only needs about 4 hours, since it gives each plenty of slack time to recover if something takes longer than planed. Technically they can work at the same time, but plumbers go first in practice because drain pipes must go downhill, and it easier for electric to go around everything in the way than to route plumbing or heating around wires. Just in this factor we have spent a couple weeks for things that could be done in a few hours.
Sometimes. The bank generally will need most of the construction time for their paperwork. Most people need to sell an existing house and that too takes time. So for the most part everyone is setup for the time it takes to build normally .
Until you get to site and things don’t quite fit, then the installer bodges it or you wait 3 weeks for the factory to make a new bit. Making stuff on site isn’t as subject to disasters.
Driving costs down is only good if quality isn't compromised too much. In Japan they can compromise quality to the point where they don't expect things to last more than 20 year. In most of the rest of the world people expect a house to last several lifetimes.
What is acceptable is an important question. I've been in houses from 100 years ago, most had significant remodels that clearly did things not anticipated originally and so it is obvious to a knowing eye where the different parts are. On the other hand, construction is expensive and the throw away culture seems wrong too. I'll let you decide where the trade off should be made.
Serious question, but what is a prefab home in the US made of? Here in Germany i think Ytong / aerated concrete would be a good choice, since it is a very good isolator already and you can get a pretty good subsidy if you build your house to a certain standard of energy efficiency. Ytong makes that pretty easy to accomplish. Here is a site in German with all of their houses: https://www.ytong-bausatzhaus.de/haeuser/
Here is a developer of prefab houses showing their plus energy house (meaning it produces more energy than it requires to live in): https://www.hanse-haus.de/en/prefabricated-houses/energy-eff... - these are not made out of aerated concrete though, they use timber panels.
They don't give (at least not that I can find) any specs to how they achieve that efficiency. 20 years ago I looked that up (for a different manufacture) and their vaunted passive house didn't meet the minimum insulation codes for where I lived in the US. The climate in Germany is a lot kinder than US's midwest, and so it probably was passive there.
