What extension drawings actually are
Extension drawings are scaled, measured plans of what you want built. They turn "a kitchen extension across the back" into something a builder can read, measure and price.
A typical set includes existing and proposed floor plans, elevations (how each wall looks from outside), and sometimes a cross-section showing heights and how the structure is put together. They're drawn to scale, so a wall that's 4 metres on the drawing is 4 metres on site.
This matters because a quote is only as accurate as the information behind it. A clear drawing set is the shared reference point that lets several builders price the same job fairly. A sketch on the back of an envelope can't do that.
In the Wirral and Liverpool area, most homeowners getting drawings prepared use an independent architectural design and drawing service rather than a builder, so the design stays separate from the build. SC Design Wirral is one such design-only service — more on that route below.
Why a rough idea isn't enough to quote
A builder pricing your extension has to work out materials, labour, time, access and dozens of smaller decisions. Each of those depends on detail a rough idea simply doesn't carry.
Say you ask for "a 3 by 4 metre rear extension with bi-fold doors". Sounds specific — but the builder still doesn't know the wall build-up, the roof type, the foundation depth, where services run, or how the new floor meets the existing one. Every gap becomes a guess.
Guesses make quotes unreliable in both directions. Some builders pad the price to cover the unknowns; others quote low and hit you with extras later. Neither helps you compare offers or budget with confidence.
Clear drawings remove most of that guesswork. They let the builder price what's actually on the page instead of pricing their worst-case assumption — which is usually the more expensive one.
Planning drawings vs build detail
This is the single most common point of confusion, so it's worth being clear. Planning drawings and building-regulations (technical) drawings are two different things, prepared for two different purposes.
Planning drawings show the size, shape, position and appearance of the extension — enough for the local authority to decide whether it's acceptable. They're about what it looks like and where it goes, not how it's built.
Building-regulations drawings (sometimes called construction or technical drawings) add the build detail: wall construction, insulation, foundations, structural elements, drainage, ventilation and so on. This is the information a builder really leans on to price and to build correctly.
So if you've only had planning drawings done, a builder can give you a rough ballpark — but they'll often need the technical set, or to make assumptions, before a quote becomes genuinely reliable. For a fuller breakdown, see our guide on planning vs building-regulations drawings.
What good drawings should show
You don't need to be technical to sense-check a drawing set before you send it out. The checklist below covers the details builders most often ask for.
Before you send drawings to a builder, check these details
- A clear scale and a north point or orientation on each sheet.
- Existing and proposed floor plans, so the change is obvious.
- Overall dimensions — length, width and height — clearly marked.
- Elevations showing every external face of the extension.
- A cross-section showing floor, wall and roof build-up where available.
- Window and door positions and sizes, including any bi-folds or rooflights.
- How the new floor level relates to the existing house and the garden.
- Notes on materials to match (brick, render, roof tiles).
- Drainage and where soil/waste connects, if shown.
Worked example: a rear extension
Imagine a single-storey rear kitchen extension on a Wirral semi. A builder reading a good drawing set should be able to answer, without phoning you: How deep does it project, and how wide? Is the roof flat, pitched or a lantern? Where do the bi-folds go, and how big are they?
They'd also want to see how the new floor meets the existing kitchen, whether a structural beam is needed where the old rear wall comes out, and where the rainwater and waste pipes run. With those answers on the page, the builder prices the real job. Without them, every line item carries a hidden "if".
Already have a set of drawings ready? Send them over and we'll quote the build for CH and L addresses.
Structural calculations: the caveat
Drawings show intent. They don't, on their own, prove the structure stands up. Where an extension removes a load-bearing wall, spans a wide opening or carries a storey above, structural calculations are usually needed.
These are produced by a structural engineer and sit alongside the drawings. They size beams, columns and foundations for the actual loads involved. A builder will often ask whether calculations exist, because a steel beam specified by an engineer prices very differently from a guess.
If you're not sure whether your project needs them, ask your designer or a structural engineer — don't assume. It's not legal or planning advice we can give here, and the right answer depends on your specific build. The point for quoting is simple: where calcs are needed, having them ready makes a quote far more solid.
The design route and the quote route
Two separate things usually need to happen: getting drawings prepared, and getting the build quoted. Keeping them distinct tends to give you cleaner pricing and clearer accountability.
For the design route, an independent architectural design and drawing service can prepare your extension drawings, planning drawings and technical detail. SC Design Wirral is a design-only service for homeowners across Wirral and nearby areas; according to their public website at the time of writing, they offer extension drawings, planning and building-regulations drawings, measured surveys and builder quotation packs. They're a useful external resource where you need drawings — you can visit SC Design Wirral to see what they cover, and our overview of SC Design Wirral explains how their work fits the quoting stage. If you'd like to picture the idea before drawings exist, a website visualiser can help you explore the look, though it doesn't confirm what's buildable.
- Do you have any drawings yet? If not, start with a design service to get a scaled set prepared before approaching builders.
- Do you only have planning drawings? You may still need technical detail or to expect assumptions in early quotes — see planning vs building-regulations drawings.
- Have a full set ready? Send it for a quote — WV Construction handles residential extensions across CH and L postcodes.
For the quote route, once you have drawings you're happy with, WV Construction can price the build from them for Wirral and Liverpool addresses. The clearer the set, the tighter and more reliable the quote — which is exactly why the detail above is worth getting right first.
Common missing details that slow quotes down
- No scale or no dimensions on the sheets.
- Proposed plan only, with no existing plan to compare against.
- Heights and floor levels left off entirely.
- No indication of roof type or construction.
- Structural openings shown but no calculations mentioned.
- Drainage and services not indicated anywhere.
What this guide does not replace
This guide explains what builders typically need to quote an extension — it is not legal, planning or structural advice. Whether your project needs planning permission, falls under permitted development, or requires structural calculations depends on your specific build, and you should check with your designer, a structural engineer, a planning professional or your local authority.
Extension drawings show design intent; they do not confirm that a scheme is buildable or that planning will be approved. Always work from a complete, professionally prepared set before committing to a build.
How this fits WV Construction’s process
WV Construction is a general building contractor working across CH and L postcodes (Wirral and Liverpool). Once you have a drawing set you're happy with, we can price the build from it — the clearer the detail, the more reliable the quote you get back.
If you already have drawings, the quickest start is to send them straight over for a builder quote from drawings, and we'll take it from there for your CH or L address.
Common questions
What drawings do I need for an extension?
At minimum, scaled existing and proposed floor plans plus elevations, ideally with a cross-section and dimensions. For the construction detail a builder needs, building-regulations drawings usually follow. The full picture is in our guide to planning vs building-regulations drawings.
How detailed should drawings be?
Detailed enough that a builder can tell the size, height, roof type, window and door positions, floor levels and how the extension meets the existing house, without phoning you. The more of those a drawing answers, the tighter and more reliable your quote will be.
Do I need structural calculations?
Often yes, where an extension removes a load-bearing wall, spans a wide opening, or carries a storey above. Calculations are produced by a structural engineer and size beams and foundations for the real loads. Ask your designer or a structural engineer if you're unsure — it depends on your specific build.
Can I get an extension quote without drawings?
You can get a rough estimate, but not a reliable fixed price. Without drawings a builder has to guess at the build-up, structure and services, which usually means padded or low quotes. Getting a scaled set prepared first is the surest way to a quote you can trust. If you'd like a quote and already have drawings, send them to WV Construction.
Who prepares extension drawings in Wirral?
Independent architectural design and drawing services prepare them for homeowners. SC Design Wirral is one design-only service covering Wirral and nearby areas; according to their public website at the time of writing, they offer extension, planning and building-regulations drawings. Our overview of SC Design Wirral explains how it fits the quoting stage.
Written by WV Construction; details about SC Design Wirral and TailoredQuote reflect their public websites at the time of writing and may change.