# What drives the cost of a custom home?

> Why price per square foot misleads, and the six things that actually move the cost of a custom house.

This is the plain-text version of https://blackstone.build/residential/journal/what-drives-the-cost-of-a-custom-home. To cite or link, use that page.

Budget. March 24, 2026.

## The short answer

Six things drive the cost of a custom home: its size and shape, the site, the exterior materials, the level of interior finish, the mechanical systems, and the work outside the house. Price per square foot hides all six, which is why two houses of the same size can differ in cost by a wide margin.

## Why is price per square foot misleading?

It divides everything by floor area, but kitchens, baths, stairs and roofs do not scale with area. A compact house with a complex roof can cost more per foot than a larger, simpler one.

Use it for a first rough range and nothing more.

## How do shape and site change the price?

Every corner, dormer and change in roof line adds labour. A steep or rocky lot adds excavation, retaining walls and longer utility runs before the house begins.

## What do the exterior materials add?

Natural stone, slate, copper and steel windows cost several times what siding, shingles and standard windows do, in both material and skilled labour. They also last far longer.

Photo: Terraces, walls and a pool are part of the budget, not an afterthought.

## Where does the interior budget go?

Kitchens and baths, cabinetry and millwork, stone, plumbing fixtures and lighting. These are the selections with the widest price range, and they are where allowances matter most.

## What is easy to forget?

Systems and site work: heating and cooling zones, generators, home automation, driveways, landscape, pools and outdoor rooms. Add design fees, engineering, permits and utility connections.

A complete budget lists every one of these from the first estimate.

## Common questions

### When will I know the real price?

When the drawings and selections are complete enough for trades to bid. Before that, you should get estimates that narrow at each design stage.

### How do allowances work?

An allowance is a budget for an item you have not yet chosen. If your choice costs more, you pay the difference. If it costs less, you keep it.

### How can I keep the cost under control?

Decide early, and price the design as it develops. Changes made on paper cost far less than changes made on site.

## The project behind this post

- [Mountain Brook Legacy Home](https://blackstone.build/text/residential/projects/mountain-brook-legacy-home): Custom Home. Custom Estate — Mountain Brook. Mountain Brook, AL.

From the [Residential Journal](https://blackstone.build/text/residential/journal).