Energy retrofit: costs by element

Facade insulation is measured against facade area, roof insulation against roof area, window replacement by number and window area. None of these has a fixed relationship to habitable area.

The divergence is substantial. A detached house of around 130 square metres of habitable area is typically calculated with roughly 150 square metres of facade, around 110 square metres of roof, about 90 square metres of ground floor ceiling and around a dozen windows. A terraced house of the same habitable area has a considerably smaller facade area; a compact two-storey house a smaller roof.

A benchmark per square metre of habitable area for an insulation measure is therefore not transferable between two buildings, not even at the same habitable area and construction period. Only element areas make the figures comparable.

The ranges are wide and source-dependent. For external insulation composite systems, different sources give 90 to 200, 120 to 200 and 180 to 300 euros per square metre. The difference lies mainly in the insulant and in whether scaffolding, render quality and junction details are included.

The relationship between effort and effect is worth noting. The facade is the most expensive measure with the largest saving. Ground floor and top floor ceilings are the cheapest measures and pay back fastest. For a cost estimate this means the order of measures is not a question of element size but of the ratio between investment and saving.

Scaffolding. For a 150 square metre facade, erection, dismantling and standing time over several weeks run to some thousands of euros net, equivalent to roughly 18 to 25 euros per square metre. Many contractors do not build this into the square metre rate.

Junction details. Adapting roof overhangs, renewing window sills, extending shutter boxes and disposing of construction waste add up to a four-figure sum on a detached house.

Reveal insulation. This is the single most important item for actual effectiveness and is frequently omitted. A substantial share of thermal bridges in facade retrofits arises at uninsulated window reveals, shutter boxes and balcony slabs. Facade insulation without insulated reveals does not achieve its calculated performance.

Airtightness. Internal and roof insulation require a continuous airtight layer. Executed poorly, moisture from inside condenses within the insulation. That is not a comfort question but a risk of damage.

Two dependencies are mandatory and are routinely disregarded in practice. Both concern the order in which measures are carried out.

Windows before facade insulation. New windows and external doors must be installed before insulation is applied, so that the insulation board can be carried a few centimetres over the frame. That overlap is the decisive detail at this junction. In the reverse order it can no longer be formed.

Bundle scaffolding operations. Facade, windows and roof all need the same scaffold. Combining the three measures into one phase means the scaffold is paid for once instead of three times. On a detached house the saving runs to several thousand euros.

These two points are the strongest argument for a retrofit roadmap before the first measure. Phased renovation without a coordinated sequence not only costs more, it permanently forecloses certain detail solutions.

Unlike any other construction measure, the investment here is matched by a quantifiable running saving. A cost estimate showing only the investment does not answer the client's question.

Three quantities therefore belong beside the benchmark. Without them it does not transfer.

The ordering by payback differs markedly from the ordering by investment. Ceiling insulation measures are by far the cheapest and pay back fastest, while the facade delivers the largest saving but carries the longest payback.

The third quantity is routinely overlooked. A price discount for energetically poor buildings has established itself in the property market and grows with rising energy costs and increasing regulation. Energy retrofit therefore acts not only through running costs but through preservation of value. For a viability appraisal that is the quantity with the longest lever and simultaneously the hardest to quantify.

Two mechanisms act on cost estimating.

The conditional upgrade obligation. Where an element is being renewed anyway, such as external render or roof covering, minimum energy requirements must be met on that occasion. Pure repair without energy upgrading is not permissible in those cases. This changes cost estimating fundamentally, because a measure planned as maintenance becomes a retrofit measure.

The unconditional upgrade obligation. Uninsulated, non-walkable top floor ceilings must be insulated. On a change of ownership, certain upgrade obligations run to a deadline measured from acquisition.

The minimum values to be met are set element by element. For external walls in existing buildings the requirement sits at a level reachable with conventional insulation thicknesses. The specific values and exemptions are covered in the article on German building regulations.

The German federal funding scheme for efficient buildings was restructured with effect from 21 July 2026. The application portals were temporarily closed beforehand and reopened on that date under new conditions.

For measures to the building envelope the grant rate remains, according to published information, at 15 per cent, rising to 20 per cent with an individual retrofit roadmap. New is that the roadmap bonus applies only to costs above a threshold. For heating replacement, which runs through a separate programme, several bonuses were reduced or removed and an income-dependent maximum rate introduced.

Two points hold irrespective of the current version. First, the application must be made before the supply or works contract is concluded; a contract already signed excludes the grant. Second, a listed energy efficiency expert must be involved.

For cost estimating a distinction follows that must be held consistently: funding does not reduce construction cost, it reduces the client's own contribution. The two belong in separate lines, because one is fixed and the other depends on a decision notice.

• Renovation costs per m² by depth of intervention
• Full refurbishment: costs by depth of intervention
• Additional costs for efficiency house and passive house standards
• Reference areas: BGF, NRF and Wohnfläche
• German building regulations and standards