How to Project Manage a Kitchen Extension UK

A kitchen extension is one of the most complex — and most rewarding — home renovation projects you can undertake. Get it right and you'll add significant value, transform how you live in your home, and create a space that works for decades. Get it wrong and you could face structural problems, planning enforcement, or a kitchen that doesn't fit the space it was supposed to fill.

Typical costs range from £30,000 for a modest single-storey extension to £70,000+ for a larger, highly specified project. This guide takes you through every phase of the project from initial brief to final sign-off.

1. Setting the Brief

Before you approach an architect or builder, define what you actually want. A vague brief produces an inaccurate quote and a design that misses the mark.

Size: How many square metres do you need? A typical kitchen extension adds 15–30m² to the ground floor. Measure your current kitchen and work out what size would genuinely transform it.

Layout: Open-plan kitchen-diner-living? Separate utility? Bifold doors onto the garden? Where is the sun at various times of day — a rear extension facing north will be darker than one facing south.

Style: Does the extension reference the existing house (traditional brick, pitched roof) or take a contemporary approach (flat roof, large glazing, render)? Your LPA's preferences and your neighbours' likely objections should inform this early.

Budget: Set a realistic figure before you talk to professionals. For a 20m² single-storey kitchen extension:

• Basic spec: £30,000–£40,000
• Mid spec: £40,000–£55,000
• High spec (underfloor heating, bespoke kitchen, structural glazing): £55,000–£70,000+

Add 15–20% contingency on top of any figure you're given.

2. Planning Permission vs Permitted Development

Many single-storey rear extensions can be built under Permitted Development (PD) rights, avoiding the need for full planning permission.

PD rights allow (for houses, not flats):

• Single-storey rear extension up to 4m depth (detached) or 3m depth (semi-detached / terraced)
• Under the Prior Approval (Neighbour Consultation Scheme), these limits extend to 8m (detached) or 6m (semi-detached / terraced) — subject to no objection from adjoining owners

You need full planning permission if:

• The extension exceeds PD limits
• Your property is in a conservation area, AONB, or National Park (some PD rights are removed)
• Your property is listed
• You've already used your PD allowance on previous extensions
• The extension affects a side elevation or extends beyond the principal elevation

Prior Approval process:

1. Submit a Prior Approval application to your LPA (currently ~£120)
2. LPA consults adjoining owners for 21 days
3. If no objection, Prior Approval is confirmed — you can build
4. If objection received, LPA assesses on planning merit (similar to full permission)

Always apply for a Lawful Development Certificate (LDC) even for PD works — this costs ~£234 and proves to future buyers and their solicitors that the work was lawful.

3. Structural Design

RSJ Calculations

Where the extension connects to the existing house, you'll need to knock through and support the load above with a steel beam (RSJ). A structural engineer will calculate the required beam size and specification. This is not optional — a beam undersized for its span is a structural risk and will fail Building Regulations.

Structural engineer fee: £500–£1,500 for calculations and drawings.

Party Wall Notice

If your property is semi-detached or terraced, any excavation within 3–6m of the neighbouring foundation, or any work on or adjacent to the shared wall, triggers the Party Wall etc. Act 1996. You must serve notice on your neighbour(s) before starting work.

• If they consent, work can proceed — get this in writing
• If they dissent (or don't respond within 14 days), a Party Wall Surveyor is appointed
• You typically pay the surveyor's fee (£800–£2,000+ depending on complexity)

Serve notice early — at least 2 months before you plan to start, as disputes add time.

Building Regulations

All extensions require Building Regulations approval, regardless of PD status. You can apply via:

• Full Plans application: Submit drawings for pre-approval before starting. Recommended — catches issues before you've committed to a build.
• Building Notice: Start without pre-approval, with inspections throughout. Higher risk if work doesn't comply.

Submit your application to your local Building Control Body (BCO) or an Approved Inspector. Fee: £400–£1,200 depending on project size.

4. Procurement

Architect

An architect adds cost (5–10% of build cost, or £3,000–£5,000 fixed for planning drawings on a standard extension) but typically delivers a better design, smoother planning process, and clearer contractor briefing. For extensions over £40,000, the cost is usually justified.

If budget is tight, an architectural technician (lower fees) or a design-and-build contractor can provide drawings, though specification detail may be thinner.

Main Contractor vs Trade-by-Trade

For most homeowners without construction experience, a main contractor on a JCT Minor Works or JCT Homeowner Contract is the lower-risk option.

Getting Quotes

Get at least three quotes from contractors who have completed similar projects locally. Ask for:

• Itemised breakdown (not just a single figure)
• Proposed programme with key milestone dates
• Details of what's excluded (provisional sums for kitchen, glazing, etc.)
• Their public liability and employer's liability insurance certificates
• References from recent jobs you can visit

5. Programme

A typical kitchen extension takes 12–20 weeks from start on site to completion. The range reflects project size, specification, and how prepared your procurement is before work starts.

Phase-by-Phase Breakdown

Total: 13–22 weeks

6. Budget Management

Contingency

Build 15–20% contingency into your budget from day one. Not as a savings buffer — as an allocated line item. Kitchen extensions regularly encounter hidden drainage runs, inadequate existing foundations, asbestos in older properties, and structural issues in adjacent walls.

Stage Payments

Never pay the full contract sum upfront. A reasonable payment structure for a main contractor:

• 10% mobilisation payment on start (covers materials ordering)
• Staged payments tied to programme milestones (foundations complete, weathertight, plastering complete)
• 5% retention held back for 3–6 months post-completion (covers any defects emerging after handover)

Variations and Provisional Sums

Provisional sums (PS) are budget allowances in the contract for items not yet fully specified at tender — typically kitchen supply, glazing, and electrical fit-out. Agree a realistic PS, then confirm the actual cost before the trade mobilises.

Variations (changes to the agreed scope) must be instructed in writing with a price agreed in advance. Verbal variations without agreed pricing are a common source of disputes at final account.

7. Building Control Sign-Off

Building control inspections happen at key stages — the BCO needs advance notice (usually 24–48 hours) before each inspection. Key inspection stages:

1. Commencement (before foundations are excavated)
2. Foundations (before concrete is poured)
3. Damp-proof course
4. Structural steelwork (before encasing)
5. Roof structure
6. Insulation
7. Final inspection

On final inspection, the BCO checks compliance with the approved drawings and issues a Completion Certificate. This is a critical document — retain it carefully. Solicitors will request it when you sell the property.

If the BCO raises issues, you (or your contractor) must rectify before the certificate is issued. Don't accept handover from your contractor until you have written confirmation that building control sign-off is imminent or complete.

8. The Most Common Mistakes

Ordering the Kitchen Before the Structure is Measured

Kitchen cabinets are manufactured to specific dimensions. Ordering from drawings before the actual opening is built and measured leads to expensive remakes or awkward infill panels. Measure the actual opening once plastering is complete.

Glazing Lead Times

Bespoke bifold doors, rooflights, and structural glazing can have lead times of 12–16 weeks from order. If you order at the start of the structural phase, they'll arrive just in time. If you forget to order until the roof is on, expect a 3-month programme extension.

Underfloor Heating and Screeding Sequence

Wet underfloor heating (UFH) is embedded in a screed layer (typically 65–75mm). The screed must cure for approximately 1 day per mm of depth before floor finishes can be laid — for a 75mm screed, that's 75 days minimum. This must be factored into your programme. Electric UFH (thinner, faster) is an alternative for retrofits.

Not Reading the Planning Conditions

Planning conditions require specific actions before or during construction (pre-commencement conditions) and at specific stages. Failing to comply with a pre-commencement condition (such as submitting a materials sample) can render the permission void. Read every condition on your decision notice.

Project Checklist

For detailed 2025 cost figures by extension depth and quality tier — including the trade-by-trade breakdown, glazing variables, and contract advice — see our kitchen extension cost UK 2025 guide. For guidance on vetting builders, writing a solid contract, and managing payments throughout the project, see our guide on how to find and manage tradespeople for a UK renovation.

• Renovation Cost Contingency: How Much to Budget and Why — kitchen extensions are high-contingency projects; this guide sets out the right bands and what the fund should and shouldn't cover
• When to Use a Project Manager vs Manage Your Own Renovation — kitchen extensions are exactly the project type where a PM's fee can pay for itself; here's how to decide

Use Renovate Me to build your personalised kitchen extension roadmap, track your programme, and manage your budget stage by stage.