I've been a chartered quantity surveyor for over 40 years. In that time, I've reviewed thousands of building cost estimates produced by contractors, architects, online calculators, and other consultancies. The majority of them are wrong. Not marginally — meaningfully wrong. And the consequences range from projects that run over budget by 20% to developments that never get off the ground because the initial numbers made them look viable when they weren't. This article is for developers, contractors, and architects who want to understand what separates a useful building cost estimate from one that will cause problems later.
They're Based on Rates, Not Reality
The most widespread shortcut in construction estimating is the use of cost-per-square-metre rates. These figures appear in publications, databases, and industry guides. They are useful for ballpark figures at feasibility stage. They are not an estimate. A rate of £2,500/m² for a residential new build tells you almost nothing about what your specific project will cost. It doesn't account for the ground conditions on your site, the specification your architect has drawn up, the height of the building, the distance from a supply chain, the complexity of the structure, or the current procurement market. An estimate is a document that reflects your project. A rate is a generalisation. Conflating the two is how budgets get set at the wrong number and projects run into trouble.
They're Produced Too Early and Never Updated
It's common for a developer to receive an estimate at RIBA Stage 1 or 2, set a budget based on it, and not update it until the project is out to tender. By that point, the design has changed, the specification has changed, material costs have moved, and the original estimate is obsolete. Cost planning needs to be iterative. The estimate at feasibility should be revisited at design development. It should be updated again at planning. It should be checked against the tender documents before they go out. An estimate that doesn't track the design is a liability.
They Don't Cover Everything
Construction costs are only one part of a development budget. A building cost estimate that covers the main contractor works but ignores external works, professional fees, planning costs, finance costs, statutory fees, and contingency isn't an estimate of what a project will cost. It's an estimate of one portion of the expenditure. I've seen projects stall because the developer budgeted accurately for construction but left no room for what sits around it. The total development cost needs to be understood from day one.
What a Good Estimate Actually Looks Like
A proper building cost estimate for a residential or commercial project should include an elemental cost plan with costs broken down by element so you can see where the budget is being consumed and where the risk sits. It should include specification notes stating what specification has been assumed, because if the cost plan assumes timber frame but the architect draws steel, the figures are wrong. It should include an exclusions list defining what is not included, as this is as important as what is. It should include a contingency allocation as an explicit percentage for design development and construction risk, separate from the base estimate. It should include a market adjustment note on current tender market conditions and how they affect confidence in the figures. And it should include a programme assumption, because costs vary with programme and a project completing in 12 months will have different preliminaries from one completing in 24 months. At Page Building Consultants, our estimates are produced to this standard for every project, from domestic extensions to £150 million commercial developments. We achieve a typical margin of error of 2–3% against final tender prices, which means the budget set from our cost plan is the budget the project gets tendered at.
When to Instruct a Quantity Surveyor
The most common mistake I see is instructing a QS too late. By the time a project reaches planning or detailed design, the major cost decisions have already been made — the structural system, the envelope specification, the floor-to-ceiling heights, the number of lifts. Changing any of these at tender stage is expensive. Getting the QS involved at RIBA Stage 2 (Concept Design) means you have cost feedback when decisions are still easy to change. For smaller residential projects — extensions, refurbishments, conversions — the calculus is different. Here, the main benefit of a QS is not early-stage cost planning but accurate tender documentation: a Bill of Quantities or detailed specification that gets you comparable quotes from contractors and protects you when the job is being built.
Related Resources
A building cost estimate is either a tool that protects your project or a liability that exposes it to risk. The difference lies in whether it reflects your specific project, tracks the design as it develops, and covers the full scope of expenditure — not just the construction works. If your current estimate doesn't meet these standards, it is probably wrong. And wrong estimates cost far more than professional advice ever will. At Page Building Consultants, we produce detailed, project-specific estimates with a typical 2–3% margin of error. Contact us for a free, no-obligation discussion — fees start from just £200 + VAT.