House Extension Timeline UK — How Long From Planning to Completion?

One of the first questions homeowners ask when considering an extension is: how long will this take? The answer depends on many variables — the size of the project, your planning authority’s workload, the contractor’s availability, and the complexity of the design. But with proper planning, realistic expectations, and good professional advice, most extensions can be completed within 9–18 months from initial brief to moving in.

This guide gives you a detailed, stage-by-stage breakdown of a typical UK house extension timeline, so you know what to expect and how to avoid the most common delays.

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Overview: Typical Extension Timeline

Stage	Duration
Architect briefing and initial design	2–4 weeks
Planning drawings and submission preparation	3–6 weeks
Planning application determination	8–13 weeks
Building regulations drawings	3–6 weeks
Contractor tender and selection	4–8 weeks
Mobilisation and site setup	2–4 weeks
Construction	8–20 weeks
Internal fit-out	4–8 weeks
Snagging and final inspections	2–4 weeks
Total	36–73 weeks (9–18 months)

This range is broad because of the variability in planning determination times and construction programmes. A simple single storey rear extension with Permitted Development (no planning required) can be completed in 5–7 months. A complex two storey extension requiring planning permission will typically take 12–18 months.

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Stage 1: Architect Briefing and Initial Design (2–4 weeks)

Your architect will visit the property, measure up, discuss your requirements, and produce initial design options. This stage involves:

• Site survey and measured drawings of the existing property
• Feasibility discussion covering planning constraints, Permitted Development status, structural considerations, and budget
• Concept design options — typically 1–2 layout options for client review
• Cost estimation — initial budget guidance based on outline design

What can delay this stage:

• Difficulty accessing all parts of the property for survey
• Indecision on brief or budget during the design phase
• Complex planning history requiring additional research

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Stage 2: Planning Drawings and Submission (3–6 weeks)

Once the design is agreed, your architect prepares a planning package:

• Existing and proposed floor plans at 1:50
• Existing and proposed elevations at 1:50 and 1:100
• Site location plan (Ordnance Survey based)
• Design and access statement (for more complex applications)
• Householder planning application submitted via Planning Portal

In some cases, a pre-application enquiry is worth submitting first — the local authority provides informal feedback on your proposals before you commit to a formal application. This adds 3–5 weeks but can prevent a refusal.

Permitted Development: If your extension falls within Permitted Development rights, you can skip the planning stage entirely and move straight to building regulations. This saves 3–4 months in many cases. Crown Architecture recommends applying for a Certificate of Lawful Development (Proposed) even for PD projects to formally confirm lawfulness.

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Stage 3: Planning Application Determination (8–13 weeks)

The statutory determination period for a householder planning application in England is 8 weeks from the date of valid application. However:

• Many applications are extended by agreement to allow additional time
• Complex or contested applications may take 10–13 weeks
• Nationally Significant Infrastructure Projects and major applications have different timescales (not relevant for residential extensions)

What happens during this period:

• The application is validated and assigned a case officer
• A site notice is posted and neighbours are notified
• Statutory consultees (highways, ecology, etc.) are consulted if relevant
• The case officer assesses the application against planning policy
• A planning committee hearing may be required if officers recommend refusal or the application is called in

What can delay this stage:

• Invalid application (missing documents, incorrect fee)
• Objections from neighbours requiring extended consultation
• Request for additional information from the case officer
• Conservation area or heritage considerations requiring specialist assessment

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Stage 4: Building Regulations Drawings (3–6 weeks)

Building regulations drawings are typically prepared in parallel with planning determination or immediately after planning approval:

• Detailed architectural drawings at 1:20 and 1:10 showing construction details
• Structural engineering calculations and drawings (designed by your structural engineer)
• Specification document covering materials, insulation standards, fire strategy, etc.
• Building regulations application submitted to LABC or approved inspector

A Full Plans application is checked by building control before work begins, providing certainty about technical compliance.

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Stage 5: Contractor Tender and Selection (4–8 weeks)

Once planning and building regulations packages are ready, your architect can compile a tender package for submission to builders. This includes:

• Architectural drawings and specification
• Structural engineer’s drawings
• Employers’ requirements or schedule of works

Typically 3–4 contractors are invited to price the work. Allow 3–4 weeks for tender return, then time for tender analysis and negotiation before appointing.

What can delay this stage:

• Contractor availability — good local builders are often booked 2–3 months ahead
• Large discrepancies in tender prices requiring re-design or value engineering
• Slow tendering from busy contractors

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Stage 6: Mobilisation and Site Setup (2–4 weeks)

Before work begins on site, several things need to be organised:

• Party wall notices (if applicable) — must be served at least 2 months before work starts
• Scaffold design and erection
• Hoarding and site security
• Material deliveries (bricks, blocks, lintels, structural steel)
• Insurance — check your buildings insurance covers the extension period

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Stage 7: Construction Programme (8–20 weeks)

Construction time varies enormously by project size and complexity. Typical programmes:

Project type	Construction programme
Single storey rear extension (4m × 6m)	10–14 weeks
Two storey rear extension (4m × 6m)	14–20 weeks
Side return extension	10–14 weeks
Wrap-around extension	16–22 weeks

Typical construction sequence:

1. Demolition of existing elements
2. Excavation and foundations
3. Drainage
4. Substructure (up to ground floor slab level)
5. External walls (blockwork, brickwork)
6. Structural steel and lintels
7. Upper floor structure (if two storey)
8. Roof structure
9. Roofing membrane and coverings
10. Windows and external doors
11. First fix (plumbing, electrics, underfloor heating)
12. Insulation
13. Plasterboard and plastering
14. Second fix (electrics, plumbing connections, joinery)
15. Tiling, painting, floor finishes

What causes construction delays:

• Bad weather during groundworks and below-ground construction
• Material supply chain issues (steel, bricks, timber)
• Hidden site conditions (tree roots, old drainage, made ground)
• Subcontractor no-shows or scheduling conflicts
• Client changes to specification mid-build

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Stage 8: Internal Fit-Out (4–8 weeks)

After the main construction is complete, internal fit-out covers:

• Kitchen installation (if applicable)
• Bathroom fitting
• Floor finishes (tiles, engineered wood, carpet)
• Painting and decorating
• Joinery (shelving, wardrobes, internal doors and ironmongery)

Specialist trades (kitchen fitters, tilers, decorators) need to be booked in advance — skilled tradespeople in high-demand areas are often unavailable at short notice.

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Stage 9: Snagging, Inspections, and Sign-Off (2–4 weeks)

Before practical completion, your architect carries out a snagging inspection to identify any defects or incomplete items. Once these are resolved:

• Building control final inspection — leading to issue of a completion certificate
• Final payment to contractor (typically the last 5% of the contract sum is held as retention)

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How to Speed Up Your Extension Project

1. Start with an architect immediately — don’t wait until you’ve saved the full budget
2. Investigate Permitted Development — eliminating planning saves 3–4 months
3. Overlap stages where possible — structural engineering and tender preparation can begin before planning is granted
4. Prepare for party wall notices early — they must be served well in advance
5. Book contractors early — good builders have long lead times

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Frequently Asked Questions

How long does a simple single storey extension take from start to finish?

From first meeting with an architect to moving in, allow 7–12 months. If your extension falls under Permitted Development (no planning required), the lower end of this range is achievable.

How long does planning permission take for a house extension?

The statutory period is 8 weeks from valid application. In practice, many householder applications take 10–13 weeks. Complex or contested applications can take longer.

What causes the most delays on extension projects?

Planning permission delays, contractor availability, and material supply are the most common causes. Good professional project management reduces all three.

Can I live in the house during an extension?

Yes, in most cases. Ground floor extensions can disrupt kitchen and living areas, so plan for this. Loft conversions and second storey additions are usually manageable to live through.

How long does building regulations take for an extension?

A Full Plans application is typically approved in 4–6 weeks. Inspections during construction add time but no fixed delay — they must be booked in advance to avoid holdups.

Is 6 months realistic for a house extension?

For a simple Permitted Development single storey extension starting from a complete design package, 6 months on site is possible. From initial brief to completion, 8–10 months is more realistic.

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Crown Architecture manages residential extension projects from inception to completion across the UK. Contact us for a free initial consultation.