Single vs Two Storey Extension London: Which Is Better?
Single storey: £60k-£90k (10-15% value increase). Two storey: £95k-£150k (15-25% value increase). Two storey costs only 50% more but adds 100% more space. Complete decision guide.
Quick Comparison
Single Storey (20m²)
- Cost: £60,000-£90,000 (£3,000-£4,500/m²)
- ROI: 10-15% value increase (£70k-£105k on £700k home)
- Timeline: 12-16 weeks build
- Often qualifies as PD (3-4m rear projection)
Two Storey (40m² total)
- Cost: £95,000-£150,000 (£2,400-£3,750/m²)
- ROI: 15-25% value increase (£105k-£175k on £700k home)
- Timeline: 16-20 weeks build
- Usually needs planning permission (max 3m rear PD)
Cost Comparison: The Math That Surprises Everyone
According to DeVis Architecture, two storey extensions cost only 50% more than single storey but deliver 100% more floor area. This is because both need the same foundations and roof structure - you're essentially "stacking" a second floor for just the additional wall and floor cost.
| Extension Type | Total Cost | Floor Area | Cost per m² | Value Added |
|---|---|---|---|---|
| Single Storey (20m²) | £70,000 | 20m² | £3,500/m² | £70k-£105k (10-15%) |
| Two Storey (20m² × 2) | £105,000 | 40m² | £2,625/m² | £105k-£175k (15-25%) |
| Difference | +£35,000 (50%) | +20m² (100%) | -£875/m² (25% cheaper) | +£35k-£70k more value |
Why Two Storey Is Better Value Per Square Metre
Shared Foundations
Foundations cost £12k-£18k whether you build one storey or two. By adding second floor on same footprint, you're spreading foundation cost across 40m² instead of 20m². Result: £300-£450/m² foundations vs £600-£900/m² for single storey.
Shared Roof Structure
Roof costs £8k-£12k for 20m² footprint. Single storey: this covers your extension. Two storey: same roof covers double the floor area. Roof cost per m² of living space halves.
Professional Fees Efficiency
Architect charges £4k-£8k whether designing single or two storey (similar drawing complexity). Structural engineer: £2k-£4k for either. Planning application: £258 flat fee. These fixed costs divided across 40m² vs 20m² = significant per-m² saving.
Additional Second Floor Cost
- •First floor structure (joists, flooring): £8k-£12k
- •Additional brick walls (20m² × 2.5m high): £10k-£14k
- •Windows, doors, finishes for second floor: £8k-£12k
- •Additional labour time (4-5 weeks extra): £4k-£6k
Bottom line: Adding second floor costs £30k-£40k extra but gives you 20m² more space. That's £1,500-£2,000/m² for the second floor vs £3,000-£4,000/m² for single storey. Two storey extensions deliver 25-35% better cost efficiency than building the same total area as two separate single storey extensions.
Property Value: Why Two Storey Wins
According to CR Design Services, two storey extensions increase property value by up to £4,000-£6,000 per m² because they add bedrooms. Single storey extensions rarely match this value increase.
The Bedroom Premium: Where Value Really Comes From
Property valuations are primarily driven by bedroom count, not total floor area. Going from 2-bed to 3-bed or 3-bed to 4-bed creates step-change value increases that far exceed the cost of construction.
Single Storey Extension (20m²)
- • Adds: Kitchen-diner or larger living space
- • Bedroom count: Unchanged (still 2-bed or 3-bed)
- • Value increase: 10-15% (£70k-£105k on £700k)
- • Buyer appeal: Modest improvement
Two Storey Extension (40m²)
- • Adds: Kitchen-diner PLUS extra bedroom
- • Bedroom count: Increases (2→3 or 3→4)
- • Value increase: 15-25% (£105k-£175k on £700k)
- • Buyer appeal: Market-transforming
Bedroom Count Premium in London
2-bed → 3-bed
£80k-£120k value jump in London (Zones 2-3). Transforms property from "young couple starter" to "young family home." Rental yield improves 15-25%.
3-bed → 4-bed
£100k-£180k value jump in desirable areas. Opens property to larger families who filter searches by "4+ beds." Premium schools become accessible with extra bedroom for second child.
Adding bedroom vs adding floor space
Extra 20m² living space (no bedroom): £3,500-£5,000/m² value = £70k-£100k increase. Extra 20m² with bedroom: £5,000-£8,750/m² value = £100k-£175k increase. Bedroom adds £30k-£75k premium.
ROI Comparison: £700k London Property
Single Storey (20m²): £70k cost
Value increase: £84k (12%)
ROI: 120% (pays for itself, £14k profit)
Good ROI but doesn't change bedroom count = limits buyer pool
Two Storey (40m²): £110k cost
Value increase: £140k (20%)
ROI: 127% (pays for itself, £30k profit)
Better ROI AND changes bedroom count = expands buyer pool significantly
Key insight: Two storey extensions deliver better ROI (120-135% vs 90-120% for single storey) despite higher absolute cost because adding a bedroom creates step-change value increases that exceed the extra £35k-£40k construction cost. If you can afford the extra £35k-£40k, two storey is almost always the smarter financial choice.
Planning Permission: Single Storey Has Advantage
According to Designs In Detail, two storey extensions have much stricter permitted development limits (maximum 3m rear projection) vs single storey (3-4m rear). Most two storey extensions need full planning permission.
Single Storey PD Rules (Easier)
- Maximum depth: 3m (terraced/semi) or 4m (detached)
- Maximum height: 4m, or 3m within 2m of boundary
- Eaves: Can be higher than existing house eaves
- Timeline: No planning needed = 3-4 months faster (16-20 weeks vs 28-32 weeks)
Result: 60-70% of single storey rear extensions qualify under PD
Two Storey PD Rules (Stricter)
- Maximum depth: 3m only (applies to all house types)
- Maximum height: 3m if within 2m of boundary (very restrictive)
- Eaves: Cannot be higher than existing house eaves
- Timeline: Planning permission needed = 3-4 months added (plus rejection risk)
Result: 80-90% of two storey extensions need full planning permission
Planning Permission Cost & Timeline Impact
Additional Costs for Two Storey:
- •Planning application fee: £258
- •Architect planning drawings: £1,500-£3,000 extra
- •Design & access statement: £500-£1,000
- •Potential planning consultant if rejection risk: £1,500-£3,000
Total extra cost: £2,758-£7,258
Timeline Comparison:
Single Storey (PD)
Design: 6-8 weeks → Build: 12-16 weeks = 18-24 weeks total
Two Storey (Planning)
Design: 6-8 weeks → Planning: 12-16 weeks → Build: 16-20 weeks = 34-44 weeks total
Planning trade-off: Two storey extensions take 4-5 months longer and cost £3k-£7k more due to planning requirements, but this delay is usually worth it for the superior value and ROI. Exception: if you're selling within 6-12 months, the timeline advantage of single storey PD becomes critical.
Decision Framework: Single or Two Storey?
Choose Single Storey If:
- Budget is £60k-£90k - can't stretch to £110k+ for two storey
- Don't need extra bedroom - already have 3-4 beds, just need bigger living space
- Timeline is critical - selling within 6-12 months, can't wait 8-10 months for two storey
- Planning rejection risk - conservation area, difficult neighbors, strict council
- Garden preservation priority - want to minimize garden loss (single storey often means less depth)
- Bungalow or single-storey property - two storey would dramatically change character
Choose Two Storey If:
- Need extra bedroom - 2→3 beds or 3→4 beds upgrade essential for family/rental
- Budget allows £110k-£150k - can afford the extra £35k-£50k for second floor
- Maximizing ROI - two storey delivers 120-135% ROI vs 90-120% single storey
- Staying 5+ years - timeline and disruption justified by long-term value gain
- Property value £600k+ - higher value properties justify the investment better
- Cost efficiency matters - two storey is 25-35% cheaper per m² than building same area single storey
Rule of thumb: If you can afford the extra £35k-£50k AND need a bedroom, two storey is almost always the better choice financially. If you don't need a bedroom or budget is tight, single storey delivers excellent value. Don't choose two storey just to "maximize space" without the bedroom - the planning complexity and timeline aren't worth it unless you're gaining bedroom count.
Frequently Asked Questions
Can I build single storey now and add second floor later?
Yes, but expensive and disruptive. Your single storey foundations may not support second floor weight (need £8k-£15k underpinning). You'll pay for scaffolding twice, disruption twice, and planning fees twice if needed. Better approach: build two storey from start if there's any chance you'll want second floor within 10 years. If truly unsure, design single storey foundations to support future second floor (adds £3k-£5k now, saves £10k+ later).
Does two storey extension affect existing house structure?
Yes, more than single storey. Additional weight requires: (1) Deeper foundations (1.2-1.5m vs 0.9-1.2m for single), (2) Structural connection to existing house first floor, (3) Potential internal wall strengthening if tying into existing structure, (4) Roof tie-in modifications. Your structural engineer will assess existing foundations - older Victorian houses sometimes need minimal strengthening if existing foundations are robust, modern houses usually handle it fine. Cost: £2k-£5k extra structural work vs single storey.
Will two storey extension make my house dark?
Potentially yes, if designed poorly. Two storey extensions block more light to existing rooms than single storey (7-8m high vs 3-4m). Solution: (1) Limit rear projection to 3-4m max, (2) Use large glazing on extension rear wall, (3) Add roof lights in extension roof to bounce light into existing house, (4) Consider step-back design (second floor set back 1-2m from ground floor). Well-designed two storey shouldn't darken house significantly. Your architect should model sunlight impact.
What if I only need 10m² more space - still build two storey?
No. Two storey only makes economic sense if you need 30m²+ total space (15m²+ per floor). Building 10m² single storey + 10m² second floor = £60k-£80k, terrible value per m². Better options: (1) Build just 10m² single storey (£30k-£40k), (2) Wait and save for proper 20m² single storey when budget allows, (3) Consider loft conversion instead if you just need one bedroom. Two storey's cost efficiency comes from spreading fixed costs (foundations, roof, fees) across 35-50m² total space.
Does single storey have better resale appeal than two storey?
No, opposite is true. Two storey extensions with extra bedroom sell faster and command higher premiums. Buyers filter property searches by bedroom count - going from 2→3 or 3→4 beds expands your buyer pool significantly. Single storey extensions are nice-to-have (better kitchen-diner), two storey extensions are market-transforming (bedroom count change). Exception: bungalows or older buyers who value single-level living - then single storey extension maintains property character and appeal to target market.
Should I do wrap-around single storey or smaller two storey?
Depends on priority: Wrap-around single (side return + rear): £95k-£130k, adds 35-45m² ground floor, no bedroom, 12-15% value increase, needs planning permission. Small two storey (rear only): £95k-£130k, adds 30-40m² across two floors, ADDS BEDROOM, 15-25% value increase, needs planning permission. Cost is similar, planning required for both. Choose wrap-around if: you have wasted side return and don't need bedroom. Choose two storey if: bedroom count is limiting factor. Two storey usually wins financially due to bedroom premium.
Summary
Two storey extensions cost 50% more (£95k-£150k vs £60k-£90k) but deliver 100% more space, making them 25-35% cheaper per square metre. They add more value (15-25% vs 10-15%) because they increase bedroom count, which drives property valuations. ROI is better: 120-135% vs 90-120%.
Trade-off: Two storey extensions need planning permission (80-90% of cases), adding 3-4 months timeline and £3k-£7k cost. Single storey extensions often qualify under permitted development (60-70% of cases), making them 3-4 months faster to complete.
Choose two storey if: Need extra bedroom, budget allows £110k+, staying 5+ years, maximizing ROI. Choose single storey if: Don't need bedroom, budget £60k-£90k, timeline critical (selling soon), planning rejection risk high.