Bands by type of intervention
| Intervention | Cost per m2 | Note |
|---|---|---|
| Ground extension, independent structure | EUR 1,300 - 1,700 | The cheapest case: behaves like new construction |
| Ground extension, structurally continuous | EUR 1,500 - 2,000 | Connections and joints with the existing building |
| Vertical addition in lightweight structure (timber, steel) | EUR 1,600 - 2,100 | Contained loads, fast site |
| Vertical addition in masonry or concrete | EUR 1,900 - 2,600 | High loads, greater checks and strengthening |
| Conversion of an existing loft | EUR 900 - 1,400 | Adds no volume: cheaper than both |
The comparison with new construction (EUR 1,600-2,000 per m2 for mid-range level, as in the article on residential construction costs) says the essential: adding volume to an existing building costs as much as or more than building from scratch, at equal floor area. The value of the operation lies in the location, not in a saving.
Why a vertical addition costs more
The structural check is mandatory and can overturn the project. Adding a storey means increasing loads on existing walls and foundations: seismic regulations require verification of the whole building, and the outcome may demand masonry strengthening, framing, foundation reinforcement. These are the least predictable costs of the project and can be worth as much as the addition itself.
The site is at height and on an occupied building. Full-height scaffolding, vertical material handling, protecting the existing building from water ingress while the roof is removed: items a ground extension does not have.
The existing roof is demolished and rebuilt. Not a detail: disposal of the covering, complete rebuilding of the frame and the insulation package fall within the addition's budget.
Services must be extended vertically. New risers, boiler upgrade, electrical sizing on the overall load.
The choice of a lightweight structure (timber or steel) responds directly to this problem: reduced loads mean checks that are easier to satisfy and less strengthening, which explains why it dominates residential vertical additions even though the structure's cost per m2 is comparable. The reduced load also limits work on the existing foundations.
The checks to complete before any estimate
| Check | What it establishes | When to do it |
|---|---|---|
| Remaining development capacity | whether the volume is achievable | first of all |
| Distances, heights, site coverage ratios | the project's dimensional limits | at feasibility stage |
| Landscape or heritage protections | admissibility and imposed materials | at feasibility stage |
| Structural and seismic verification | feasibility and required strengthening | before the estimate |
| Condition of the foundations | the worst possible cost | with opening-up, before the estimate |
The order matters: the planning check precedes the structural one, because a volume that cannot be built makes every calculation pointless. And the structural check precedes the estimate, because it is the only item that can multiply the budget.
Example: a 70 m2 timber vertical addition
A 1980s detached house, one storey added in timber structure, mid-range finishes. The example includes the structural checks and the connecting works.
| Item | Calculation | Amount |
|---|---|---|
| Demolition of existing roof and disposal | EUR 12,000 | |
| Timber structure, floor and new roof | 70 m2 × EUR 900 | EUR 63,000 |
| Finishes, windows, services of the new floor | 70 m2 × EUR 750 | EUR 52,500 |
| Strengthening of existing structure (estimated post-verification) | EUR 28,000 | |
| Scaffolding and temporary works | EUR 9,000 | |
| Services upgrade and connections | EUR 11,000 | |
| Professional fees (15%, including structural design) | EUR 26,300 | |
| Contingencies (12%) | EUR 21,000 | |
| Indicative total | EUR 222,800 | |
| Full cost per m2 | / 70 | ~EUR 3,180 |
The full cost per m2 (EUR 3,180) far exceeds the starting band: the item that makes the difference is the strengthening, which no preliminary estimate can quantify without verification. That is why in this type of project the serious estimate arrives after the structural calculation, not before.
Note: the figures shown are 2026 orders of magnitude for the Italian market, for guidance only. Every extension or vertical addition requires a planning check, structural calculation and a specific estimate.