Lets break it down.
Single vs double storey extension costs in Ireland, including cost per m² comparison, structural differences, total project budgets and efficiency factors.
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.
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.
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.
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:
Greater efficiency per m²
Roof cost spread over larger usable floor area.
Below are realistic construction cost ranges for house extensions in Ireland based on current labour, material and compliance conditions.
Basic shell finish
Typical Cost per m² (Single Story)
€2,100 – €2,500
Typical Cost per m² (Two-Story)
€2,200 – €2,700
Standard finish
Typical Cost per m² (Single Story)
€2,600 – €3,200
Typical Cost per m² (Two-Story)
€2,400 – €3,000
High specification
Typical Cost per m²
€3,300 – €4,200
Typical Cost per m²
€3,100 – €3,800
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.
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.
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 |
Cost Breakdown Comparison – 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.
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.
Single Storey Vs Double Storey Price Comparison
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.
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.
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 |
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.
Other resources:
Extension Cost per Square Metre
Structural Steel and Foundation Costs
Timeline & Staged Payments Breakdown