How does the cost calculator work?

The Havnwright cost calculator is a guided property assessment, not a flat lookup table. It produces a range, not a single number, and the range gets tighter as you answer more questions.

What you tell us

1. Property type: detached, semi, terrace or flat.
2. Project type: kitchen, bathroom, extension, loft, full refurb, etc.
3. Size: small / medium / large.
4. Quality tier: Essential, Quality or Luxury.
5. Location: postcode prefix (used for regional cost adjustments).

That gives you an initial estimate range. Then you can refine by answering more property-specific questions (era, condition, layout). Each answer either tightens the range or surfaces a real cost the initial estimate didn't include.

Where the prices come from

• Base materials and labour rates are taken from verified UK trade sources, updated quarterly.
• Regional pricing uses ONS regional construction indices. Work in London or the South East costs more than the North.
• VAT is shown separately so you can see the gross and net figures.
• A contingency percentage is built in based on project type.

Why your number can go up after refinement

Refinement does not always lower the cost. If your answers surface conditions that need extra work (poor electrics, sloping floors, period property), the engine adds those costs honestly up front rather than letting them appear later as variations.

This is the point: a higher number you can trust beats a low number that gets blown out mid-project.

How accurate is it?

The accuracy meter at the top of the results page shows where you are: General → Good → Strong → Very Strong. To get to "Very Strong" you need to complete the refinement questions. Even at that level, real quotes from contractors will land within (or sometimes a little above) the range, never far below.

See also: What is a Scope of Works?