How to Get an Accurate Building Estimate for Your Extension Project

Whether you're planning a rear extension, a new build, or a commercial development, getting an accurate building estimate is the single most important step before breaking ground. An inaccurate estimate can derail your entire project — causing budget overruns, contractor disputes, and costly delays.

At Page Building Consultants, we've produced thousands of estimates for projects ranging from £10,000 domestic extensions to £150 million commercial schemes. Here's exactly what you need to know.

What Information Do You Need to Provide?

The quality of your estimate is directly tied to the quality of information you provide. The more detail you can share upfront, the more accurate and reliable your estimate will be.

• Architectural drawings or sketches — even rough plans help significantly
• Site location and access details (affects labour and material delivery costs)
• Specification level — standard, mid-range, or high-end finishes
• Structural requirements — any known ground conditions or existing building constraints
• Programme timeline — when you need the work completed
• Any specialist requirements — underfloor heating, smart home systems, bespoke joinery

What Should a Professional Estimate Include?

A professional building estimate is far more than a single number. It should break down costs by trade and element so you can understand exactly where your money is going and make informed decisions about where to save or spend more.

• Preliminaries — site setup, welfare facilities, project management
• Substructure — foundations, groundworks, drainage
• Superstructure — frame, external walls, roof
• Internal finishes — plastering, flooring, ceilings, decorating
• Mechanical and electrical — heating, plumbing, electrics, ventilation
• External works — landscaping, driveways, fencing
• Contingency allowance — typically 5–10% for unforeseen works
• Professional fees — architect, structural engineer, planning

How Long Does an Estimate Take?

At Page Building Consultants, we pride ourselves on speed without sacrificing accuracy. For most domestic projects, we can turn around a detailed estimate within 24 hours of receiving your drawings and specification.

For larger commercial or mixed-use schemes, we typically require 3–5 working days to produce a full bill of quantities or cost plan. We'll always confirm the turnaround time when you submit your enquiry.

Common Mistakes to Avoid

Over the years, we've seen the same mistakes crop up time and again. Here are the most common pitfalls when getting a building estimate — and how to avoid them.

• Accepting the cheapest quote without understanding what's included
• Not accounting for VAT — most construction work is subject to 20% VAT
• Ignoring contingency — unexpected costs are almost inevitable on any project
• Failing to get a fixed-price contract before work starts
• Not checking the contractor's insurance and accreditations
• Underestimating professional fees — these can add 10–15% to your total project cost

Why Use a Professional Estimating Service?

Many clients ask whether they really need a professional estimating service, or whether they can just get quotes directly from contractors. The answer depends on your project, but for anything above £50,000, a professional estimate is almost always worth the investment.

A professional estimate gives you an independent, unbiased view of what your project should cost. It puts you in a much stronger negotiating position with contractors, helps you identify overpriced tenders, and gives your lender or investor the confidence they need to fund your project.

Page Building Consultants has been providing professional building estimates across the UK for over 38 years. Our team of qualified quantity surveyors and estimators brings real-world construction knowledge to every project we work on.