A square is the most economical shape to build for anything under 32 ft. square. Above that, a rectangle is more economical. The roof and foundation are the most expensive shell components, with the exterior walls coming in last. These big 3 determine the basic cost more directly than your interior layout.
Design on a 24-inch framing module. Everytime you can place a door or window opening within that module you save on framing costs. At least butt one side of each opening up against a stud on the module. Use 24-inch on center framing (called "Optimum Value Engineering" or "Advanced Wall Framing") for your exterior framing (16-inch o.c. for the roof for strength). This also gives you more space for insulation and reduces thermal bridging because you have fewer wood studs--reduces your costs to heat/cool.
In TX I would think a slab on grade is readily built, but you have some unstable soils, which can make a slab slide, so check with informed builders there. If you can use it, the slab on grade is cheapest. Here in GA you also get the lowest property tax rate for that foundation. Check with your local tax assessor's office for that info. You need to consider your taxes, utility bills down the road; not just the cost to build.
Have your builder use roof trusses rather than stick-built roof framing. Most do now. Allow your builder or direct your designer to play with the roof overhang and roof pitch a little. A small change can optimize your materials use for both sheathing (very expensive) and framing lumber.
Keep your roof simple. Omit dormers and excess gables and all of those that are non-functional.
If you're building a 2-story, stack the framing so it lines up vertically.
Place your large openings (garage doors, patio door, wide windows) in nonbearing gable ends to minimize header and wall-framing.
As to the interior, the fewer walls, the lower the cost. You'll get to interior finishes later, which is usually about 40% of your total cost, but a lot of us don't want to shave money off there, because that's what we value most. Save your money on your shell.
There are a million more ideas, and your builder or designer won't tell you most of them because it is not their concern, generally, to reduce your costs. Get a copy of Fernando Ruiz' book titled Building An Affordable House. It is based on his work with production builders, so you won't want to do all the things in there, but his designing the basic shell for economy is PRICELESS!
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end glass cabinet lighted love that.
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