Extension Plans : what they are, when you need them, and how to get them right first time.
Thinking about extending your home and keep seeing the phrase extension plans everywhere? This guide walks you through, step by step, what those plans actually include, when they’re needed for planning permission and building regulations, how much they tend to cost in the UK, and how to avoid common council and builder headaches.
Quick summary: extension plans (UK)
- What they are: Scaled drawings showing your existing house and the proposed extension – used for planning permission, building control and builders’ quotes.
- When you need them: Almost any house extension will need proper plans, even under permitted development or where only building regulations approval is required.
- Typical cost: For a modest single-storey project, many homeowners spend somewhere between £800–£1,800+ for planning drawings, and more if a full building regs package and structural design is needed.
- Timescales: From measured survey to a planning-ready set of extension plans usually takes around 2–4 weeks, plus the council’s 8-week decision period.
- Key checks: Planning rules, neighbours, drains, structure, and compliance with Parts A, B, L and other building regulations must all be factored into the design.
Not sure what level of extension plans you actually need?
Our team at Plans Made Easy can review your ideas, flag planning risks, and explain the drawing options in plain English before you spend money.
When you start looking into a home extension, one of the first things you’ll be asked for is a set of extension plans. Councils, builders and building control all rely on these drawings, so getting them right at the start makes the whole project smoother and far less stressful.
In this guide we’ll explain what proper extension plans include, when they’re required for planning permission and building regulations, how much they usually cost in the UK, and the step-by-step process to get them drawn up without nasty surprises.
What are extension plans for UK house extensions?
In simple terms, extension plans UK are scaled drawings that show your existing house and the new extension in enough detail for the council, an engineer, building control and your builder to understand exactly what is being proposed.
They’re usually produced by an architect, architectural technologist or experienced designer after a measured survey of your property. Good plans become the backbone of the whole project – from planning application through to the final completion certificate.
Types of drawings typically included in extension plans
A standard set of house extension plans for planning permission will often include:
- Existing floor plans – your house as it stands today.
- Proposed floor plans – showing the new extension layout, walls, doors, windows and room uses.
- Existing and proposed elevations – front, rear and side views, so the council can see height, roof form and materials.
- Sections – sliced “through” the building to show heights, levels and structure.
- Site or block plan – your property in relation to boundaries, neighbours and the street.
- Roof plan – helpful where the extension roof changes the existing structure.
For building regulations, extension plans often go further, adding insulation build-ups, beam sizes (once a structural engineer is involved), fire protection details and notes on ventilation, drainage and foundations.
Extension plans and planning permission rules
Many homeowners first encounter extension plans when they’re told they need drawings for a planning application. Whether you are applying under full planning or relying on permitted development rights, you still need clear plans that meet the council’s validation requirements.
If you’re unsure whether your project actually needs formal permission, it’s worth reading a broader guide on how the UK planning permission system works overall and specifically the rules for extensions before commissioning expensive drawings.
Can you build an extension without planning permission?
Some rear and side extensions fall under permitted development, so there is no need for a full planning application. However, the rules are strict on height, depth, percentage of garden covered and distance to boundaries. Councils still usually expect proper extension plans when you apply for a Lawful Development Certificate or when they check whether the work is compliant.
Our separate guide on when extensions do and don’t need formal permission goes through the size limits in more depth. This is important because drawing an extension that accidentally exceeds those limits could trigger a full planning application you weren’t expecting.
Why planners insist on clear extension plans
Planning officers rely on your extension plans to answer questions such as:
- Will the extension overshadow or overlook neighbours?
- Does it sit comfortably with the existing house and street scene?
- Is the height, bulk and roof form in line with local policies?
- Are there any obvious issues with parking, access or trees?
If your drawings are unclear, inconsistent or missing key measurements, the application may be delayed, declared invalid, or refused. Taking a little extra time to prepare a good quality set of extension plans often saves months of frustration later.
Step-by-step: how to get extension plans drawn
The process for getting extension plans in place is broadly similar across most of England and Wales, although each council has its own quirks. Here’s a straightforward route most homeowners follow.
1. Clarify what you want from the extension
Start by listing the problems you’re trying to solve – cramped kitchen, lack of storage, no decent family space, or a need for a home office. Sketch a rough layout by hand if you like, but don’t worry about scale at this stage.
It also helps to look at our wider guide to house extensions to sense-check what type and size of addition typically works well for your style of property.
2. Check key constraints early
Before paying for detailed extension plans, consider:
- Whether your home is in a conservation area or is a listed building.
- The position of public sewers (see our guide on building over sewers).
- Access for construction and whether a party wall notice might be required.
- Local design guidance from your council.
These factors can all affect the size and design of your extension, so your plans should take them into account from day one.
3. Choose who will prepare your extension plans
Most homeowners use one of four routes:
- Architects – typically more design-led and used for larger or complex projects.
- Architectural technologists or technicians – often very experienced in domestic extensions and technical drawings.
- Design-and-build companies – combine drawings with construction, but check who actually owns the plans.
- Online extension plans services – can be cheaper, yet sometimes less tailored to local planning policies.
4. Measured survey and first draft
Your chosen designer will usually visit to take measurements and photos of the existing house. They then draw this up as the “existing” base plan and start working on layout options for the proposed extension.
It’s normal to have at least one round of tweaks at this stage, adjusting room sizes, doors, glazing and roof form until you’re happy the extension plans meet your needs and your budget.
5. Planning-ready extension plans and submission
Once you’ve agreed a final scheme, your designer prepares the planning set of drawings: scaled plans, elevations, sections and location plans, usually with a short design statement. These are uploaded with the application forms to the Planning Portal or your local council’s system.
If you want help specifically with the application side, our article on drawings for a planning application explains exactly what most councils expect to see.
6. Building regulations and technical drawings
After, or sometimes in parallel with, the planning process, additional extension plans are prepared for building regulations. These technical drawings show insulation, structure, fire safety, drainage and other details needed either for a Building Notice or Full Plans submission.
What should good extension plans include?
While every project is different, most well-prepared extension plans will cover:
- Accurate floor plans of existing and proposed layouts, with dimensions.
- Clear elevations showing heights, materials and window positions.
- Sections through key areas, especially where levels change.
- Site plan with boundaries, neighbouring windows and garden space.
- Roof design, including any rooflights, lanterns or parapets.
- Notes on external finishes, such as brick, render and roof tiles.
For technical and building regulations plans, you would then expect extra information such as U-values (see our guide to building regulations U-values), ventilation routes, steel beams, lintels and drainage runs.
How much do extension plans cost in the UK?
One of the most common questions we hear is, “How much do extension plans cost?”. There’s no single figure, because fees depend on location, complexity and who you choose to draw them, but there are some useful ballparks.
Typical design and drawing costs
- Smaller single-storey extensions: many homeowners pay in the region of £800–£1,800 for planning drawings.
- Larger or two-storey extensions: costs can rise to £1,500–£3,000+ where more design work and negotiation with planners is needed.
- Full technical and building regs package: adding specifications and details for building control can add another £700–£2,000, plus separate structural engineer fees.
Some designers charge a fixed fee, while others work on percentage of build cost. Either way, it’s useful to compare fees with the overall expected extension build cost so you can see where the drawings sit in the bigger picture.
How long do extension plans take from start to finish?
Timescales vary by designer and council workload, but a typical route for UK homeowners looks like this:
- Initial discussions and quote: a few days to a week.
- Measured survey: usually within 1–2 weeks of instruction.
- First draft extension plans: around 1–3 weeks after survey.
- Revisions and final planning set: another 1–2 weeks depending on how many changes are needed.
- Planning decision period: normally 8 weeks for householder applications once validated.
- Technical and building regs drawings: 2–4 weeks, sometimes overlapping with the planning process.
Where projects are straightforward and homeowners are decisive, extension plans can move quickly. However, if you’re changing your mind often or if the council raises concerns, allow more time.
Extension plans and building regulations approval
Even where planning permission isn’t needed, building regulations almost always still apply to extensions. That means your extension plans must show that the work complies with Parts A (structure), B (fire safety), L (conservation of fuel and power) and other relevant sections.
You can learn more about the approval routes in our guide to building regulations approval, and how inspectors work in practice in our article on building control inspections.
What extra information is needed for building regs extension plans?
A building regulations set of drawings for an extension often adds:
- Details of foundation type and depth.
- Wall build-ups, insulation thicknesses and cavity barriers.
- Floor and roof construction, including joist sizes.
- Fire safety measures, such as escape windows and fire doors.
- Drainage layouts, manholes and connection to existing services.
Many homeowners will also need structural calculations for beams and openings. These are usually produced by a structural engineer and referenced on the extension plans so building control can check everything together.
Common mistakes with extension plans (and how to avoid them)
We often see similar issues crop up when homeowners first arrange extension plans. Being aware of them early can save both time and money.
- Inaccurate base survey: if the existing house is drawn incorrectly, every later drawing is also wrong.
- Ignoring drains and manholes: moving or building over sewers can need separate permission from the water company.
- Overlooking neighbouring windows: this can lead to objections and refusal on privacy or light grounds.
- Not thinking about structure early: large openings may need substantial beams, which affect ceiling heights and costs.
- Poor natural light planning: deep extensions without rooflights or wider openings can feel dark and gloomy.
- Rushing straight to builders: asking for quotes without consistent extension plans often leads to wildly different prices.
A short review with someone who understands both planning policy and construction – which is exactly where our team sits – can help spot these problems before your plans go anywhere near the council.
FAQs: extension plans for UK homeowners
Do I need plans for an extension?
In practice, yes. Almost any extension that involves new foundations, structure or changes to the exterior will need proper extension plans. Councils expect scaled drawings for planning or lawful development applications, and building control needs technical information to check compliance. Even if you are building under permitted development, it’s risky to rely on rough sketches alone.
Can I draw my own extension plans?
It is legally possible to draw your own house extension plans, and some simple projects are done this way. However, you must still meet the council’s drawing standards and show all the required information. If you’re not comfortable with scaled drawings, measurements, levels and planning policy, using a professional designer usually saves time and reduces the risk of refusal.
How much do extension plans cost compared with the build?
For typical domestic projects, design and drawing costs are often a small percentage of the overall extension budget. While every case is different, spending sensibly on good extension plans usually pays off through smoother approvals, clearer builder quotes and fewer changes on site. If you’re unsure what’s reasonable, compare fees with the likely build cost using resources such as our breakdown of extension costs.
Are extension plans different for two-storey or side extensions?
The basic drawing principles are the same, but two-storey, wraparound or side extensions often raise more planning issues around overshadowing, overlooking and street character. Extension plans for these schemes usually need more careful design work and may involve extra sections, streetscene views or planning statements to justify the proposal.
Do extension plans cover heating, electrics and finishes?
Planning-stage extension plans normally focus on layout, size and appearance rather than detailed services. Technical building regulations drawings may include information on heating, ventilation and insulation performance, but the exact wiring layout and final finishes are often agreed directly with your builder and electrician. Make sure any important preferences – such as underfloor heating or large rooflights – are clear on the drawings from the start.
What happens if my extension is built differently to the approved plans?
If the built extension doesn’t match the approved extension plans, the council can ask for changes, a new application or, in serious cases, enforcement action. Building control may also refuse to sign off work that doesn’t align with the drawings they checked. Always speak to your designer and the council before making significant changes once work has started.
Next steps: making extension plans work for you
Extension plans aren’t just a box to tick for the council. They’re a chance to design a home that genuinely works better for your day-to-day life, while keeping planners, neighbours and building control on side.
If you’re still at the early ideas stage, you might find it helpful to read our step-by-step guide to planning a property extension the right way alongside this article. When you’re closer to pressing go, our resources on what’s included in extension drawings and plans for building regulations approval will help you choose the right level of detail.
Need clear advice on your own extension plans?
Share your rough sketch and postcode with our team at Plans Made Easy.
We’ll outline which permissions you’re likely to need, what drawings are required, and the typical costs and timeframes – all in plain English, before you commit to a full design fee.
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