Introduction
One of the most common sources of confusion in the early stages of a building project is the difference between types of drawings. Homeowners are sometimes surprised to find that the drawings submitted for planning permission are not the same drawings needed to price or build the project. Understanding this distinction early — before you start collecting quotes — can save a significant amount of time, money, and frustration.
What Are Planning Drawings?
Planning drawings (sometimes called architectural or design drawings) are produced to support a planning application. Their purpose is to communicate the design intent to the local planning authority — showing what the proposed extension or loft conversion will look like and how it fits within its context. Typical planning drawing sets include existing and proposed floor plans, elevations, a site location plan and block plan, and section drawings showing the relationship between floors and roof.
Planning drawings are usually drawn to illustrative rather than construction standard. They are sufficient for a planning authority to assess the principle of development — but they do not contain the technical information a contractor needs to build, or the calculations a structural engineer needs to produce their specifications.
What Are Construction Drawings?
Construction drawings (also called technical or working drawings) are produced to communicate how a building will be constructed. A construction drawing set might include detailed floor plans with room dimensions, door and window schedules, technical cross-sections showing floor build-up, insulation specification, and wall construction, foundation details developed from structural engineer input, steel beam positions and padstone details, information on drainage runs and connections, and specification notes for materials and finishes.
Why Using the Wrong Drawings to Price Causes Problems
A quote based on planning drawings is inherently approximate. The contractor must make assumptions about foundation type and depth, steel beam size and specification, wall construction and insulation detail, and drainage arrangements. When those assumptions turn out to be wrong — or when the structural engineer's actual specification requires something different — the price changes. This is not the contractor being dishonest; it is the inevitable consequence of pricing against incomplete information.
What You Should Prepare Before Getting Quotes
For a meaningful, comparable set of quotes, you ideally need architectural construction drawings, a structural engineer's design or brief, a clear specification of finishes (or an agreed assumption that second-fix materials are client-supplied), and drainage information. The more complete the information package, the more accurate and comparable the quotes will be. If complete construction drawings don't yet exist, be explicit about this when requesting quotes and ask each contractor to state what assumptions they have made.
Design-and-Build as an Alternative
In a design-and-build model, the contractor takes responsibility for producing or coordinating the construction information as part of the overall project. This is often more efficient than the traditional route — because the contractor brings buildability input into the design process rather than inheriting problems after the fact. RCB Design & Build helps clients develop the right level of information before seeking a final price — so that when the contract is signed, the scope is based on real data rather than assumptions. Contact us to review your drawings and advise on what additional technical information is needed.