The Gap Between a Ballpark and a Detailed Quote
An early ballpark figure is a broad estimate based on limited information. It might be based on a brief phone conversation, a rough project description, or a comparison with similar past projects. It is useful for early-stage planning, but it is not a price.
A detailed quote is based on actual drawings, a proper site survey, structural information, specification of materials, and confirmed scope. When all of that information is available, the price becomes much more reliable. The gap between the two is the gap between a rough idea and a real project.
Common Reasons Extensions Cost More Than Initial Estimates
1. Structural complications. Many extensions involve removing or modifying load-bearing walls, creating openings for bifold or sliding doors, or carrying beams across spans that require significant steelwork. Until a structural engineer has reviewed the specific scheme, the steel requirement is often an assumption.
2. Ground conditions and foundations. Foundation type is one of the most variable elements of any extension. Soft ground, high water tables, proximity to trees, or older existing structures can all require more expensive foundation solutions — deeper strips, pad foundations, or even piling.
3. Drainage and sewer build-over. An extension that sits over or near a public sewer may require a build-over agreement with Thames Water. This has both a time and cost implication that is easy to miss at the early planning stage.
4. Design development. When a client's design evolves between the initial quote and the detailed design — more glazing, a higher-specification kitchen, a different internal layout — the cost changes with it.
5. Finishes and specification. Early estimates often assume a mid-range standard of finish. When clients choose high-specification tiles, bespoke joinery, underfloor heating, or premium kitchen units, the cost of those elements differs from the assumption.
6. Services upgrades. Older properties may need electrical upgrades, new consumer units, upgraded boilers, or changes to heating systems as part of the extension project.
What You Can Do About It
Get proper drawings before requesting a detailed quote. Ask what is included — and what is not. Build a contingency of 10–15% into your budget. Have a structural engineer involved early. Treat the ballpark as a range, not a fixed figure.
At RCB Design & Build, we separate ballpark figures from detailed estimates and are clear about what each one is. Our ballpark figures include a written disclaimer explaining what they are based on, what is not yet known, and why the detailed figure may differ.
Want a properly structured estimate that explains what is included and what might change? Contact RCB Design & Build: 07359 872594 | contact@rcbgroup.co.uk