Let building what you want be the "dog" and the builder be the "tail". Plans offered by builders are in most all cases bad unless significant changed.

It sounds like you need a custom builder and an architect. Edited to add: Looks like I posted same time as MB. Have the architect design the house then take the plans to custom builders for estimates on build cost.

Builders do not design home plans. They build them.

If you are using a builder with plans you are not getting a custom home. The builder controls all and you are just paying them. If you own the land, hire an architect/designer and get a design, then bring it to builders to quote on you get more say and control. There is no one way to build a home but you need to know what you want to get out of it. If you want to build this summer I doubt you can do a custom home with that little notice.

You'll likely incur additional costs for design and for construction of a unique, custom home. An alternative to the ABC (architect before contractor) approach which you may wish to consider is design/build. Design/build firms have competencies in both design and construction. The design fees are typically less with design/build than following the ABC approach. Plus, you'll have the benefit of the builder's cost estimating throughout the design process which reduces the chance you'll pay for a set of plans that never get built.

Keep in mind when you provide plans to a builder you warrant them as free from errors and omissions. In the event there are errors and omissions (there almost always are) you'll be responsible for the cost to remedy them.

There is a reason builders/developers repeat the same floor plan. It's really good and lots of people like it and find it functional. When the buyers go to sell, the new buyers like it too. You and your architect may like your new design, but don't be surprised when the marketplace doesn't vindicate your choices.

We frequently see builder designs here that have none of those qualities. I often renovate homes and am frequently apalled by what has been built. But I can't complain because it brings me a lot of work and its fun fixing things that are broken..

I once designed a major modification to a townhouse (one of 6) that was under construction. The developer used my design for the next set of townhouses. Any design can be improved and should be personalized.

Instead of fostering a senseless debate about who should design a house, post the builder's designs. There are members here that live to redesign people's floor plans. Floor plan issues are too repetitive for me; I like to fix the exteriors.

The OP's question is with regard to incremental costs if they provide the plans to their builder.

" I once designed a major modification to a townhouse (one of 6) that was under construction. The developer used my design for the next set of townhouses."

I hope the builder paid you for it. Classic case of ideas having value.

I’m not an expert but it seems a builder who uses plans over and over may have costs fairly pinned down and know for sure what that looks like.

If you provide your own plan it would be more likely costs would go up and efficiency down as they haven’t worked with the plan before. Not that you won’t get good value still but just a thought.

Yes, in almost all cases.

But that's how you get more of what you want in a design.

Thanks everybody for your input. One of the builders that we are interviewing is offering that we can use their designer/architect to come up with our own plan. The cost is 5k but if we build with him the 5k gets refunded to us. GC said he has worked with that designer for years. I'm thinking going that route may keep what we want on budget since GC and designer have long relationship, they can communicate and designer would know what's cost efficient with the builder we are working with.
We want nothing crazy. Basically we just want to utilize the square footage that would've been for a dinning room and an office (typical plans) into a bigger kitchen with eating area and a 4 season room.

A builder offering to incentivize a build using a plan they already have in the binder doesn't at all guarantee that the plan is any good on its own merits, only that the builder is familiar with it.

Consider the "premium" you pay for bringing your own plan to the table a discount being taken away for inconveniencing the builder, because that's actually what it is.

"One of the builders that we are interviewing is offering that we can use their designer/architect to come up with our own plan."

If you hire me I will sing and play the violin at dinner at your favorite restaurant.

(Same effect)

(If you haven't guessed by now, I can not sing or play the violin)

Each approach to new construction has its own merits and drawbacks and every approach can create value for you. Personally, I am not a fan of production builds as I want some specific spaces and layouts that don't have wide appeal.

One pertinent question is how do you plan to finance the build? If this is a traditional 20% down and finance the remainder then you should stick with a production builder that has an appraisal guarantee. If there are significant funds to devote towards a build then custom designed becomes a good strategy.

The next question is about your long-term plans. If you are looking for your forever home then again, an architect and a custom build make more sense. If this is a transitional home for you, then production builds may be better.

Builder's existing plans are typically those with which the builder is already familiar and experienced. While some adjustments may be possible, usually these are limited.

I agree with bry911: it all depends on your reason for building, except to reinforce that if this is a "forever" home you may benefit in working with an experienced architect to get a home designed specifically for your living style and preferences.

Bringing in an experienced general contractor early in the design process often helps control costs and maintain budget.

Good luck!

their designer/architect to come up with our own plan. The cost is 5k but if we build with him the 5k gets refunded to us.

The last time I paid $5K for a plan was about 1989. High design that aint!