Houses that are the same as ones across town are cheaper as the builder has experience with it and knows the tricks to build it with less labor. They also are designed with years of feedback about what other like and dislike. They don't have weird corners that are not usable because after putting things in that is what fit.

how is it then the case that there are multiple versions of products on the market that differ very little from each other?

Let's say you want to sell plastic brooms. With such a common product, what possible competitive edge could you get by creating your own design from scratch? If your reason for getting in the business is that you want to address a niche or trend (e.g. you want to sell hot pink brooms because it's Barbie summer of 2023), that can be accomplished with an existing design. It only makes sense to create new molds for a broom if you're a corporation like Ikea with the massive sales volume that enables vertical integration of every aspect of the product.

You subcontract to various manufacturing companies — companies also being used by your competitors. Those companies have standard "best practice" ways of doing certain things, and they tell you that your design will be executed a lot more cheaply if they're allowed to nudge it toward conforming with those best practices, so that they can use standard parts and techniques.

Iterate this three or four times — especially in the context of a company that's plateaued in market share and is now all about cost optimization — and eventually you have a version of your product which is indistinguishable from your competitors' products. That's what allows for the best economies of scale up the pipeline, and therefore saves you and your competitors the most money.

In a way it's like forking a software project: the more you deviate from what's done before, the more 'pain' you'll be in.

What's done before can be re-done by any competent builder.

But beyond that, you'll need expertise in different fields, to fit parts of a design together in new ways.

No expertise -> project will fail, or be extremely 'painful' (like: costly, time-consuming, many iterations, result isn't what you wanted, etc).

Enough expertise -> steps in new territory might be uncomfortable or include some mistakes, but overall you'd know where you're at, where you're going & have realistic expectations.

So yeah - nothing wrong with getting some expert help when entering new territory.

That assumes part availability, though. There were kit-built octagon-shaped homes in the 1800s; but good luck asking a builder to build you one today.

Great analogy, seriously. As a big fan of that program (cathartic if you have any real state project going on) let me add that I have seen programs in which the house is not finished after 8 years; some in which rooms projected for kids are not needed because they are going to college; and couples that start together, and by the end of the program are divorced (or one of them passed) by the end of the program. It really puts things into context.

Also, my favourite, when 80% of the participants casually say they want to be in by Christmas, summer vacations, New years next year, and the host smiles and says something like (let's see).

That's the thing that I would expect here: that the firm you'd hire would come with a built-in person to look at your design and say "no, this isn't gonna work, try again."

Reputable product manufacturers actually do have that person.