Resource guide | 10 min read
Extension Plans & Architectural Drawings Guide for UK Homeowners 2026
Planning a home extension is one of the most significant investments a UK homeowner can make. Before a single brick is laid, you need the right architectural drawings — not just for planning permission, but for building regulations approval, contractor pricing, and structural sign-off. This guide explains everything you need to know about extension plans, who draws them, what they include, and how the planning process works in the UK in 2026.
What Are Extension Plans?
Extension plans are a set of architectural drawings that describe your proposed home extension in detail. They are used for:
Without proper drawings, your project cannot legally begin in most cases.
- Submitting a planning application to your local council
- Applying for building regulations approval from building control
- Getting contractor quotes — builders need detailed plans to price the work
- Obtaining a structural engineer's calculations
- Managing the build on-site
Types of Home Extension
Single Storey Rear Extension
The most common type. Adds a room to the back of the house — often a kitchen-diner or living space. Under permitted development rights, you can extend up to 3 metres (semi-detached/terraced) or 4 metres (detached) to the rear without full planning permission in many cases, subject to conditions.
Plans required:
- Existing and proposed floor plans
- Existing and proposed elevations (front, side, rear)
- Site/block plan
- Section drawings showing roof structure
Double Storey Extension
Adds two floors to the rear or side. Almost always requires full planning permission. More complex drawings required including structural calculations.
Side Return Extension
Uses the narrow side passage that runs alongside terraced and semi-detached houses. Popular in London. Usually single storey. May fall under permitted development or require planning permission depending on size.
Wraparound Extension
Combines a rear and side return extension into one L-shaped space. Typically requires full planning permission.
Over-Structure Extension
An extension built on top of an existing single storey element (such as a garage or flat-roof extension). Requires structural assessment and usually full planning permission.
What Drawings Are Included in Extension Plans?
Planning Drawings (for the planning application)
Planning drawings typically include a location plan (1:1250 scale) showing the site in relation to surrounding streets; a site/block plan (1:500 scale) showing the property boundary and outline of proposed development; existing floor plans showing the current layout; proposed floor plans showing the new layout after extension; existing elevations showing current external appearance; proposed elevations showing how the extension will look externally; section drawings as cross-sections through the building; and a roof plan showing the existing and proposed roof.
Building Regulations Drawings (for building control)
Building regulations drawings are more detailed than planning drawings and include:
- Structural details (foundations, beams, lintels)
- Insulation and energy efficiency specifications
- Drainage details
- Party wall details (if applicable)
- Fire safety provisions
- Ventilation details
Working Drawings (for contractors)
Detailed construction drawings used on-site by builders and trades.
Do I Need Planning Permission for My Extension?
Not always. In England, many extensions fall within permitted development rights (PDR), meaning you do not need to apply for full planning permission.
Common PD allowances (England, 2026):
- Rear extensions: up to 3m (semi/terraced) or 4m (detached), single storey
- Side extensions: up to half the width of the original house
- Height limits: must not exceed the ridgeline of the original house
When PD Does Not Apply
PD does not apply if:
- You are in a conservation area or Article 4 Direction area
- Your property is a listed building
- Previous extensions have used up your PD allowance
- The extension is over PD size limits
Devolved Nations and Lawful Development Certificates
In Wales, Scotland, and Northern Ireland, permitted development rules differ — always check with your local planning authority.
Lawful Development Certificate (LDC): Even if you believe your extension falls under PD, we recommend obtaining a Lawful Development Certificate from your council. This confirms the extension is lawful and protects your position when you sell.
Tip: Crown Architecture can advise on your PD entitlement and handle LDC applications on your behalf.
The Extension Planning Process
Step 1 — Initial consultation and site survey
An architect visits your property, discusses your plans, and measures the existing building.
Step 2 — Design and drawings
The architect prepares the planning drawings based on your brief and the council's local planning policy.
Step 3 — Pre-application advice (optional)
Some councils offer a pre-application service where you can get informal feedback before submitting. Useful for complex sites or conservation areas.
Step 4 — Submit the planning application
The planning application is submitted online via the Planning Portal. Most householder applications cost £206 (England, 2026). The council has 8 weeks to decide (13 weeks for major applications).
Step 5 — Building regulations
Once planning is approved, separate building regulations drawings are prepared and submitted to building control (either the council or an approved inspector). Building control inspects the work at key stages.
Step 6 — Construction
The builder starts work. The architect can provide site visits and administration services during construction.
Step 7 — Completion certificate
Building control issues a completion certificate confirming the work complies with building regulations. This is important for insurance and for selling the property.
How Much Do Extension Plans Cost?
Extension drawing costs vary depending on the size and complexity of the project. As an indicative guide: a single storey rear extension typically runs £800–£2,500; a double storey extension £1,500–£4,000; a side return extension £900–£2,500; a wraparound extension £2,000–£5,000; and a complex or listed building project £3,000–£8,000+.
These costs cover planning drawings only. Building regulations drawings cost extra, typically 30–50% on top of planning drawings.
Crown Architecture transparent pricing: visit our pricing page for the current fee guide.
Party Wall Considerations
If your extension is near or on the boundary with a neighbour's property, the Party Wall etc. Act 1996 may apply. You may need to serve a party wall notice on your neighbour before work begins.
Extensions commonly triggering party wall obligations:
- Rear extensions up to or near the boundary
- Side extensions on or near the boundary
- Excavations within 3 or 6 metres of a neighbour's foundations
Scaffolding for Extensions
Most extensions require scaffolding during construction, particularly for:
- Roof work
- Upper floor extension work
- Work close to the boundary
Checklist: What You Need Before Starting an Extension
Before starting an extension, work through this checklist:
- Site survey completed
- Planning drawings prepared by qualified architect
- Planning application submitted (or PD confirmed)
- Lawful Development Certificate obtained (if PD route)
- Building regulations drawings prepared
- Structural calculations completed
- Party wall notices served if required
- Contractor obtained and priced from proper drawings
- Building regulations inspections booked
Related routes
Continue to the service pages most relevant to this topic
These links move readers from research into the service pages that best match the project stage they are in now.
Extension plans
Drawing packages for single- and double-storey extensions.
Architectural drawings
Planning and building regulation drawing packages.
Building regulation drawings
Technical drawings for building control.
Structural engineer
Steel beam and structural calculations.
Project pricing
Indicative fees.
FAQ
Questions homeowners often ask next
Do I need an architect for an extension?
Not legally, but using a qualified architect or architectural technologist produces better designs, avoids planning refusals, and gives you proper building regulations drawings.
How long does planning take?
Householder planning applications take around 8 weeks for a decision. Building regulations approval is ongoing during construction, with inspections at key stages.
Can I extend under permitted development?
It depends on your property type, location, and size of extension. Listed buildings, conservation areas, and Article 4 Direction areas restrict PD. Crown Architecture can advise on your specific entitlement.
What happens if I extend without planning permission?
The council can issue an enforcement notice requiring you to demolish the extension. It also causes serious problems when you sell, as buyers' solicitors will flag the lack of consent.
How much do extension plans cost?
Planning drawings for a single storey rear extension typically cost £800–£2,500, while double storey and wraparound extensions can range from £1,500 to £5,000+. Building regulations drawings are usually 30–50% on top.
Ready to talk through your project?
Planning an extension?
Crown Architecture & Structural Engineering Ltd provides full extension planning and drawing services across London, Kent, Essex, Surrey, and Hertfordshire. Call +44 7950 114633 or email info@crownarchitecture.co.uk for a free consultation.