In brief
For an extension or a storey addition (Anbau & Aufstockung) in prefab construction, expect around €60,000 (20 m² simple living-space extension) to €260,000 (60 m² storey addition with bathroom and upmarket fittings) in 2026, excluding ancillary construction costs. Per square metre, the costs usually range between €2,500 and €4,000, since the connection to the existing building, the structural work and the transitions are more elaborate than in a new build. The big plus of prefab construction: the elements are built in the factory, installation on the existing house takes only a few days, and the family can largely stay living in the house during the build. Both routes almost always require a permit — you should have the development plan, boundary distances and above all the load-bearing capacity of the existing structure checked in advance.
What a Extension / storey addition as a prefab home costs – the overview
With the Extension / storey addition as a prefab home, prefab construction pays off above all in the price: a binding fixed price ex works protects against nasty surprises, the short build time lowers the interim financing costs and the high energy standard gives you access to KfW funding in 2026. It is precisely these three levers that keep the calculation predictable.
The wall and ceiling elements are built to the millimetre in weather-protected factory halls, and the shell often stands on the building site after just one to three days. Less time on the building site means less weather risk – and that also lowers the price in the end.
This speaks in favour on price
- Binding fixed price – calculable from the start
- Short build time reduces the interim financing
- High energy standard secures KfW funding
- Price can be adjusted deliberately via the finishing stage
This drives the price up
- An upmarket selection of finishes can raise the per-m² price considerably
- The plot and incidental building costs always come on top
- Subsequent floor-plan changes cost a lot
Putting per-square-metre costs and the total price 2026 in context
The price for a Extension / storey addition as a prefab home depends decisively on the finishing stage: turnkey, the per-square-metre costs in 2026 are around 2,000 to 3,500 EUR/m², as a shell house from about 1,300 EUR/m². From €60,000 is a realistic starting price for the turnkey basic version.
In addition to the pure house price, factor in the incidental building costs (around 15–20 % of the construction sum), earthworks and a buffer. With our cost calculator you can determine your individual total price. How each price factor works is shown on the Prefab house prices 2026 page.
For an extension and storey addition in prefab construction, reckon on €60,000 to €260,000 in 2026 — depending on area, execution and fittings. The lateral extension needs its own foundation, the storey addition sits on top of the existing structure and saves it. The following table shows non-binding price ranges for 2026 by area:
| Area | Extension (single-storey) | Storey addition (one storey) | With bathroom/kitchen |
|---|---|---|---|
| 20 m² | €60,000 – 80,000 | €55,000 – 75,000 | €80,000 – 100,000 |
| 30 m² | €80,000 – 105,000 | €72,000 – 95,000 | €105,000 – 135,000 |
| 45 m² | €110,000 – 145,000 | €100,000 – 135,000 | €145,000 – 185,000 |
| 60 m² | €145,000 – 190,000 | €130,000 – 175,000 | €190,000 – 260,000 |
Non-binding ranges for 2026, excluding ancillary construction costs (10 to 15 percent for structural engineer/architect, permit and connection work). With the storey addition, reinforcements of the existing structure, a new staircase connection and scaffolding often arise; with the lateral extension, an own foundation. Calculate a breakthrough to connect with the existing structure separately. In almost all federal states, extension and storey addition require a permit.
