Understanding Home Extensions: What’s Right for Your Property
Home extensions remain one of the most popular ways to add space and value to UK properties in 2026. Rather than facing the stress and expense of moving, homeowners are choosing to extend — and it’s easy to see why.
According to government planning guidance, the right extension can add 15-20% to your property’s value whilst giving you the extra space your family needs. But get it wrong, and you’ll face planning headaches, cost overruns, and structural issues that’ll haunt you for years.
Let’s cut through the confusion and give you the real-world knowledge you need to plan, budget, and execute a successful home extension.
Types of Home Extensions: Comparing Your Options
The type of extension you choose depends on your property layout, budget, and planning constraints. Here’s what most homeowners consider:
Single-Storey Extensions
Single-storey rear extensions are by far the most common. They’re ideal for creating open-plan kitchen-diners or extending living spaces into the garden. Most qualify for permitted development rights, meaning no planning permission needed.
- Typical size: 3m-6m depth, full width of property
- Cost: £1,200-£2,000 per m² (£24,000-£50,000 for typical 25m² extension)
- Build time: 2-4 months from groundworks to final plastering
- ROI: 10-15% property value increase
The plastering work on a single-storey extension involves both internal walls and ceilings. You’ll typically need multi-finish plaster over plasterboard for walls, with particular attention to thermal efficiency where the extension meets the existing building.
Two-Storey Extensions
Adding a second floor gives you double the space but comes with higher costs and usually requires full planning permission. These work well for side returns or where you need both ground floor living space and additional bedrooms.
- Cost: £1,800-£2,500 per m² (£90,000-£150,000+ typical)
- Build time: 4-7 months
- ROI: 15-25% property value increase
- Planning: Almost always requires permission
Side Return Extensions
Popular in Victorian terraced houses, side return extensions fill in that narrow alley space between your house and the boundary. They’re perfect for enlarging kitchens without extending too far into your garden.
Typical dimensions: 1.2m-2m wide, extending 3-5m back. Costs run £25,000-£50,000 for single-storey, significantly more if combining with rear extension.
Wrap-Around Extensions
Combining side and rear extensions creates an L-shaped wrap-around that dramatically transforms ground floor space. These are substantial projects requiring experienced builders and careful coordination between trades.
| Extension Type | Typical Size | Cost per m² | Total Cost Range | Planning Required? |
|---|---|---|---|---|
| Single-storey rear | 20-30m² | £1,200-£2,000 | £24,000-£60,000 | Usually no (PD) |
| Two-storey rear | 40-60m² | £1,800-£2,500 | £72,000-£150,000 | Yes |
| Side return | 8-15m² | £1,500-£2,200 | £12,000-£33,000 | Often no (PD) |
| Wrap-around | 35-50m² | £1,400-£2,100 | £49,000-£105,000 | Usually yes |
| Loft conversion | 25-40m² | £1,600-£2,800 | £40,000-£112,000 | Depends on dormer |
Planning Permission vs Permitted Development: What You Actually Need
This is where most homeowners get confused. The good news? Many extensions don’t require planning permission thanks to permitted development (PD) rights.
Permitted Development Rules 2026
Under current Planning Portal guidelines, you can build without permission if your extension meets these criteria:
- Detached houses: Single-storey rear extensions up to 4m deep
- Semi-detached/terraced: Single-storey rear extensions up to 3m deep
- Height limits: Maximum 4m for flat roof, 4m to eaves for pitched roof
- Side extensions: No more than half the width of original house, single-storey only
- Materials: Should match existing building appearance
When You Definitely Need Planning Permission
You’ll need to submit a full planning application (£258 fee in 2026) for:
- Two-storey extensions of any size
- Extensions exceeding permitted development limits
- Properties in conservation areas or listed buildings
- Flats or maisonettes (PD doesn’t apply)
- Extensions that would result in over 50% ground coverage
- Any extension with balconies, verandas, or raised platforms
Planning applications take 8-13 weeks typically. Budget an extra £1,500-£3,500 for architect drawings and £500-£1,200 for structural engineer input.
Building Regulations: The Non-Negotiable Requirements
Every extension requires Building Regulations approval — no exceptions. This ensures your extension is structurally sound, thermally efficient, and safe.
The Building Control approval process costs £800-£1,500 for typical extensions and involves inspections at key stages:
- Foundation inspection: Before concrete pour
- Damp-proof course: Checking DPC levels and membrane placement
- Drainage: Before backfilling trenches
- Pre-plaster inspection: Insulation, electrical, heating roughed-in
- Final inspection: Completed work with all finishes
Part L Energy Efficiency Requirements
Part L regulations in 2026 are stringent. Your extension must achieve U-values of:
- Walls: 0.18 W/m²K or better
- Roof: 0.15 W/m²K
- Floor: 0.18 W/m²K
- Windows/doors: 1.4 W/m²K
This typically means 100mm PIR insulation in walls, 150mm+ in roofs. The insulated plasterboard we install usually has 50-75mm insulation bonded to 12.5mm board.
Real-World Extension Costs: The Complete Breakdown
Let’s talk actual money. These are realistic 2026 costs based on current Kent and Southeast England pricing. Expect 10-15% less in Northern England, 15-25% more in London.
Budget Breakdown for 25m² Single-Storey Extension
| Item | Cost Range | Notes |
|---|---|---|
| Groundworks & foundations | £4,000-£6,500 | Strip foundations, 1m deep typically |
| Brickwork & blockwork | £5,500-£8,000 | Includes DPC, lintels, airbricks |
| Roof structure & covering | £4,000-£7,000 | Flat roof with warm deck insulation |
| Windows & doors | £2,500-£4,500 | Bi-fold doors cost £2,000+ alone |
| Electrics (first fix & second) | £2,000-£3,500 | New consumer unit if required: +£800 |
| Plumbing & heating | £2,500-£4,000 | Underfloor heating adds £1,200-£2,000 |
| Plastering (walls & ceiling) | £1,800-£2,800 | £35-£45/m² for board and skim |
| Flooring | £1,500-£3,000 | Insulation, screed, finish |
| Decorating | £1,200-£2,000 | Mist coats, undercoat, top coat |
| Building Control fees | £800-£1,200 | Mandatory for all extensions |
| Contingency (10%) | £2,600-£4,200 | Always budget this — you’ll need it |
| Total | £28,400-£46,700 | Mid-spec finish, no major complications |
Hidden Costs That Catch People Out
These aren’t always in the builder’s quote but you’ll definitely pay for them:
- Party Wall Agreement: £700-£1,200 per neighbour if extension touches boundary
- Skip hire: £250-£400 per skip (you’ll need 2-3 for typical extension)
- Scaffolding: £800-£1,500 for 8-12 weeks
- Structural engineer: £500-£1,200 for calculations and steel beam specs
- Kitchen relocation: £2,000-£5,000 temporary kitchen during build
- Making good existing house: £1,500-£3,000 replastering where old meets new
From a plastering perspective, the junction between old and new buildings is critical. We see issues with plaster cracking along these lines if not properly detailed with expansion beads and fibre tape.
The Extension Build Process: What Happens When
Understanding the sequence helps you plan around disruption and coordinate trades properly.
Typical 3-Month Single-Storey Extension Timeline
| Week | Stage | What’s Happening |
|---|---|---|
| 1-2 | Site setup & groundworks | Excavation, foundations poured, BC inspection |
| 3-5 | Brickwork & blockwork | Walls built up, DPC installed, lintels positioned |
| 5-6 | Roof structure | Joists/flat roof deck, insulation, membrane, covering |
| 6-7 | Windows & external doors | Fitted and sealed, making building weathertight |
| 7-8 | First fix trades | Electrics, plumbing, heating pipework, no finishes yet |
| 8 | Insulation & boarding | Wall insulation, plasterboard fixed, BC pre-plaster inspection |
| 9 | Plastering | Skim coat applied, 2-3 week drying time needed |
| 10-11 | Second fix & flooring | Sockets, switches, radiators, flooring laid |
| 11-12 | Decorating & snagging | Mist coats on new plaster, final decoration, fixes |
| 12 | Final BC inspection | Sign-off and completion certificate issued |
Plastering Considerations for Extensions
As professional plasterers, we see specific challenges with extension work that don’t arise in standard refurbishments.
Thermal Bridging at Junctions
Where your new extension meets the existing building, thermal bridging can cause condensation and mould if not properly detailed. We use thermal laminate plasterboard with integrated insulation to maintain the thermal envelope.
Standard 12.5mm plasterboard won’t meet Building Regs here. You need either:
- Celotex PL4025 (50mm PIR + 12.5mm board) for U-value 0.18 W/m²K
- Kingspan K118 (75mm PIR + 12.5mm board) for U-value 0.13 W/m²K
- British Gypsum ThermaLine Plus (62.5mm total thickness) for U-value 0.17 W/m²K
Movement Cracks Between Old and New
Extensions settle slightly — it’s normal. To prevent visible cracks, we install EJ10 expansion beads at the junction. These 10mm plastic beads accommodate 3-4mm movement without the plaster cracking through.
For ceiling level changes (common where extension ceiling height differs from existing), we use shadow gap trim rather than trying to create a seamless finish that’ll crack within months.
Plaster Specifications for Extensions
For walls, Thistle Multi-Finish remains the standard — we apply 2-3mm thickness over taped plasterboard joints. Coverage: 10.4m² per 25kg bag at 2mm thickness.
Ceilings get the same treatment but many clients now request acoustic performance. British Gypsum SoundBloc boards (15mm thickness) combined with acoustic mineral wool between joists gives Rw 46dB sound reduction — important if adding bedrooms above living spaces.
Choosing Your Builder: Red Flags and Green Lights
Get the builder wrong and your extension becomes a nightmare. Here’s what to look for:
Green Lights (Good Signs)
- Detailed written quote: Itemised costs, not just a total figure
- Portfolio of completed extensions: With verifiable addresses for reference checks
- Insurance certificates: Public liability (£5m+) and employer’s liability
- Building Control familiarity: They should know the inspection schedule cold
- Fixed payment schedule: Tied to completion stages, not cash in advance
- Written contract: FMB or similar standard contract, not a handshake
Red Flags (Walk Away)
- Requires large upfront deposit (>15% is suspicious)
- Can’t provide previous client references
- “I know a building inspector who’ll pass it” — illegal and dangerous
- Pushes you to skip Building Control inspections
- Quote significantly below others (20%+ cheaper often means corner-cutting)
- No written contract or vague scope of works
- Pressure to start immediately without proper planning
Adding Value: Which Extensions Give Best ROI?
Not all extensions return equal value. Nationwide’s house price data shows these typical returns:
| Extension Type | Typical Cost | Value Added | Net Return |
|---|---|---|---|
| Kitchen extension (single-storey) | £35,000 | £45,000 | +£10,000 (29% ROI) |
| Two-storey rear (extra bed + living) | £80,000 | £95,000 | +£15,000 (19% ROI) |
| Side return infill | £28,000 | £32,000 | +£4,000 (14% ROI) |
| Garage conversion | £18,000 | £20,000 | +£2,000 (11% ROI) |
| Conservatory | £22,000 | £18,000 | -£4,000 (negative) |
Important caveat: These returns assume you’re not over-developing for your street. Adding a £100,000 extension to a £250,000 house in a street where nothing sells above £320,000 means you’ll never recover that investment.
Common Problems and How to Avoid Them
We’ve worked on hundreds of extensions. These issues come up repeatedly:
Underestimating Costs
Budget creep is real. The £30,000 extension quote doesn’t include:
- Upgrading your entire electrical consumer unit (often required): £1,200-£2,000
- New boiler if existing can’t cope: £2,500-£4,000
- Making good where kitchen removed: £2,000+
- Landscaping to restore garden: £3,000-£8,000
Always add 15-20% contingency. We’ve seen too many projects stall when money runs out at the plastering stage.
Poor Weather Protection
Extensions take 2-4 months. That’s 2-4 months where weather gets into your existing house through the opening. Builders should install temporary weatherproofing (heavy plastic sheeting, timber screens) overnight and during rain.
Water damage to existing plasterwork can add £2,000-£5,000 in repairs if not properly managed.
Ignoring Drainage
Your extension creates more roof area = more rainwater. Your existing drainage might not cope. Building Control will check this.
New soakaway costs £800-£1,500. Connection to mains drainage requires Water Authority approval (£350-£600 fee) plus physical connection work.
Frequently Asked Questions
Do I need planning permission for a single-storey extension?
Most single-storey rear extensions don’t need planning permission if they’re within permitted development limits: 4m deep for detached houses, 3m for semi-detached or terraced properties. The extension must be single-storey with maximum height of 4m, no balconies, and use materials similar to the existing house. However, you’ll always need Building Regulations approval regardless of whether planning permission is required. Properties in conservation areas or listed buildings have different rules and typically need permission.
How much does a 20m² extension cost in 2026?
A 20m² single-storey extension costs £24,000-£40,000 in most of the UK, with Southeast England and London 15-25% higher. This breaks down to approximately £1,200-£2,000 per square metre for standard specification including groundworks, structure, roof, windows, electrics, plumbing, plastering, and basic finishes. Premium specifications with bi-fold doors, underfloor heating, and high-end finishes push costs to £2,200-£2,800 per m². Always budget an additional 15% contingency for unexpected issues like poor ground conditions or making good to existing structure.
How long does it take to build a house extension?
A typical single-storey extension takes 10-14 weeks from groundworks to completion. Two-storey extensions need 16-24 weeks. The process includes 1-2 weeks groundworks, 2-3 weeks brickwork, 1 week roofing, 2 weeks first-fix trades, 1 week plastering (plus 2-3 weeks drying time), then 2-3 weeks second-fix and decoration. Weather delays are common — expect 2-4 weeks additional time if building through winter. Planning permission adds 8-13 weeks before any building work starts. The plastering stage is crucial and shouldn’t be rushed; new plaster needs proper drying time before decoration.
Can you extend a house without planning permission?
Yes, through permitted development rights. You can extend without permission if within specific limits set by the Planning Portal: single-storey rear extensions up to 4m deep (detached) or 3m (semi/terraced), side extensions up to half the original house width, and certain loft conversions without large dormers. However, these rights don’t apply to flats, listed buildings, or properties in conservation areas. You still need Building Regulations approval for all extensions. Some local authorities have removed permitted development rights through Article 4 directions — check with your council planning department before assuming permission isn’t needed.
What’s the difference between Building Regulations and planning permission?
Planning permission deals with how your extension looks, its impact on neighbours, and land use — it’s about external appearance and whether your extension fits the local area. Building Regulations ensure your extension is structurally safe, thermally efficient, accessible, and meets fire safety standards — it’s about construction quality and technical compliance. You can need one, both, or occasionally neither. Most extensions need Building Regulations approval even when planning permission isn’t required. Building Control inspects at key stages (foundations, drainage, insulation) and issues a completion certificate essential for future property sales.
Should I hire an architect for my extension?
For straightforward single-storey extensions within permitted development, an experienced builder’s drawings may suffice — expect to pay £800-£1,500 for technical drawings. However, hire an architect (£2,500-£6,000 for full service, 8-12% of build cost) if you need planning permission, have a complex design, want maximum space efficiency, or the extension involves structural alterations to the existing house. Architects add value through better design, navigating planning objections, and specifying materials correctly. They’ll coordinate structural engineer input and ensure Building Regulations compliance. For two-storey extensions or projects over £60,000, professional architectural input usually pays for itself.
Final Thoughts: Is an Extension Right for You?
Home extensions make financial sense if you’re staying in your property for 5+ years and current house prices in your area support the investment. Moving costs £25,000-£40,000 when you factor in stamp duty, estate agent fees, removal costs, and legal fees.
An extension avoids those costs whilst giving you the exact space you need. But success depends on:
- Realistic budgeting with 15-20% contingency
- Choosing an experienced builder with verifiable track record
- Understanding planning and Building Regulations before starting
- Preparing for disruption (it’s messier than you think)
- Not over-developing for your street value
The plastering stage typically happens 8-10 weeks into your project. It’s one of the final trades before decoration, and quality here determines how your extension looks and performs for decades. Poor plastering shows immediately — hollow sounds, visible joints, uneven surfaces, and cracking within months.
Whether you’re planning a modest single-storey kitchen extension or an ambitious wrap-around transformation, take time to plan properly. Speak to Building Control early, get at least three detailed quotes, and don’t cut corners on specifications. Your extension will either be your home’s greatest asset or your biggest regret — the difference is in the planning and execution.
For professional plastering services throughout your Kent extension project, contact our experienced team. We work closely with builders, architects, and homeowners to deliver flawless plaster finishes that stand the test of time.