Toilet model and number, bathtub model and number, interior door style and finish, trim type and finish, window type/manufacturer/material, roofing color and type, siding material, color and type, exterior trim material and type, floor material, tile type, counter top material, appliances and finishes. the more exact specifications you have the better and more accurate pricing you can receive.

Read every build thread you can find here. Millworkman is right; some of this cost depends on how custom your custom-build is and what sort of build contract you choose. ANd even if you know every single thing right now down to the item numbers...most of the people on these threads end up paying more than they expected, for various reasons. Start a new thread or rename your thread to something like, How much over budget was your custom build and why? And that will give you ideas to help you ward off some of those price variances.

Having specificity on the level or quality of certain finishes will be very helpful. This can be something as simple as noting what cost per square foot you want budgeted for flooring, or as specific as picking all your appliances beforehand. Cabinets can be a huge cost driver - for example, the type of wood we wanted was double the cost of what the vendor had bid. Fortunately, we caught this before beginning. Even then, after we started construction and got into detailed cabinet design, the cost went up by over 10%. The more specific you can get, the better. It's also a good idea to have at least 10 to 15% of extra budget for going over what the initial bids are. If you find that you can't afford any overages before you even started the project, then I would not proceed.

If new home builds around you are going for $500 a square foot, you are not going to be under $300. You might be up the $750 though.

The specifications go way deeper than having a shower instead of a tub. Is it a prefab acrylic base and walls? Corian base and walls? Schluter pan with 2" marble hex and marble subway walls? That's just 1 element. Every room is jam packed with details that need pricing out.

Kitchens and bath are the most expensive, after the shell itself. What is the shell being built with? A block basement and 2x6" stud walls above? ICF for all? Slab on grade, with 2x4 framing? What insulation choice? How installed? What windows? That's a college level course, right there, to pick windows. Everything is, actually, to get enough details to get a real picture of a cost.

That's why many builders use "allowances" as placeholders, until specific materials are selected. It's easier. It's also less precise. But it's a trade off between fully specifying a bid and starting 3 years later, with out of date pricing that has gone up 30%, or starting off in 6 months, with some unknowns. The bigger priced things shouldn't be unknowns. That's where to spend your initial effort.

Write down your lifestyle wishes: where you like to have coffee in the morning, where you like to have dinner, if you entertain and how many, cosy or expansive, if you have clothes enough to fill these silly walk-in closets I see now, if you think the bathroom should be an emporium or, if you like lots of outside light, if you really need a study, flex room, office, media room and gym, the list goes on. If you have a hard time putting these into words, visit some homes and figure out why you like one over the other. Oh, and decide your real budget.

How custom is custom? Will you be in a neighborood with an existing builder(s) or in the country or an infill lot? Architect? Design/build?

Get a feel for the going rates based on new homes for sale with similar finishes. Expect yours to be more than that - as what you pick for YOU will almost always be more than what a builder picks for the "market".

The range of cost here is kinda limitless. We will need to know more.