Single vs double storey extension costs in Ireland, including cost per m² comparison, structural differences, total project budgets and efficiency factors.
1. Overview
The decision between a single-storey and double-storey extension is primarily structural and financial.
While double-storey extensions typically cost more in total, they can be more cost-efficient per square metre.
However, complexity, structural integration, and internal fit-out significantly influence final cost.
2. Cost per Square Metre Comparison
Typical 2026 Cost Ranges
| Extension Type | Standard Finish €/m² | High Spec €/m² |
|---|---|---|
| Single Storey | €2,600 – €3,200 | €3,300 – €4,200 |
| Double Storey | €2,400 – €3,000 | €3,100 – €3,800 |
Double-storey extensions often show lower €/m² figures because:
- Roof cost is distributed across two floors
- Foundations are shared between floors
- Scaffolding is required regardless of floor count
- Fixed costs are spread more efficiently
However, the total contract value is higher.
3. Total Project Cost Comparison
Example Scenario: 40m² Single vs 80m² Double Storey
| Scenario | Floor Area | €/m² | Construction Cost |
|---|---|---|---|
| 40m² Single Storey | 40m² | €3,000 | €120,000 |
| 80m² Double Storey | 80m² | €2,700 | €216,000 |
Although €/m² is lower for double-storey, total project cost nearly doubles.
4. Structural Differences That Impact Cost
Foundations
Both extension types require foundations.
However, double-storey extensions often require:
- Wider footings
- Increased load-bearing capacity
- Additional structural engineering design
This may increase foundation cost by 10–20% compared to single-storey.
Structural Steel
Steel requirements are usually higher in:
- Large single-storey rear kitchen extensions with wide openings
- Double-storey extensions involving removal of load-bearing walls
Single-storey extensions with large open-plan kitchen layouts can sometimes require more steel than modest two-storey builds.
Roof Construction
Single-storey:
- Full roof area must be constructed
- Roof cost accounts for approximately 10–15% of total build
Double-storey:
- Roof cost spread over larger usable floor area
- Greater efficiency per m²
5. Planning Considerations
Single-storey extensions:
- Often qualify under exempted development (subject to limits)
- Lower visual impact
- Fewer overlooking concerns
Double-storey extensions:
- More likely to require full planning permission
- Greater impact on neighbours
- Height and overshadowing considerations
Planning requirements can extend project timeline and increase professional fees.
6. Internal Fit-Out Implications
Single-storey extensions commonly include:
- Kitchen
- Dining area
- Living space
These areas often include high-cost finishes (kitchen units, glazing, flooring).
Double-storey extensions often include:
- Bedrooms
- Bathrooms
- Storage
Cost per m² may appear lower because upper floors may not include premium finishes.
7. Cost Breakdown Comparison
Single-Storey (Typical 40m² Example)
| Category | Approx Cost |
|---|---|
| Foundations & structure | €36,000 |
| Roof | €15,000 |
| Windows & glazing | €18,000 |
| Services | €12,000 |
| Finishes | €20,000 |
| Overheads & margin | €19,000 |
| Total | €120,000 |
Double-Storey (Typical 80m² Example)
| Category | Approx Cost |
|---|---|
| Foundations & structure | €70,000 |
| Roof | €22,000 |
| Windows | €30,000 |
| Services | €25,000 |
| Finishes | €40,000 |
| Overheads & margin | €29,000 |
| Total | €216,000 |
Note how roof cost does not double proportionally.
8. Timeline Differences
| Stage | Single Storey | Double Storey |
|---|---|---|
| Construction duration | 10–14 weeks | 14–20 weeks |
| Structural complexity | Moderate | Higher |
| Disruption level | Moderate | Higher |
Double-storey builds often require longer structural sequencing.
9. When Single Storey Becomes More Expensive per m²
- Extensive glazing
- Structural knock-through of entire rear wall
- High-end kitchen installation within contract
- Complicated roof lanterns or vaulted ceilings
These can push single-storey extensions above €4,000/m².
10. When Double Storey Is More Economical
- Need for additional bedrooms
- Efficient rectangular footprint
- Minimal internal reconfiguration
- No major steel removals internally
Cost efficiency improves when layout remains structurally simple.
11. Example Full Budget Including VAT & Fees
40m² Single Storey
| Item | Cost |
|---|---|
| Construction | €120,000 |
| VAT (13.5%) | €16,200 |
| Professional fees | €8,000 |
| Contingency (7%) | €8,400 |
| Total Project Budget | €152,600 |
80m² Double Storey
| Item | Cost |
|---|---|
| Construction | €216,000 |
| VAT (13.5%) | €29,160 |
| Professional fees | €15,000 |
| Contingency (7%) | €15,120 |
| Total Project Budget | €275,280 |
12. Summary
Single-storey extensions:
- Higher €/m²
- Lower total cost
- Simpler planning
- Often kitchen-focused
Double-storey extensions:
- Lower €/m²
- Higher total cost
- Greater structural complexity
- More cost-efficient for larger space needs
The correct choice depends on required space, site constraints, and total budget capacity.
13. Other resources:
Extension Cost per Square Metre
Structural Steel and Foundation Costs
Timeline & Staged Payments Breakdown