Designing in existing buildings takes more work
A conversion does not start with a blank sheet. The architect has to survey the existing building, assess its load-bearing capacity and components, connect old and new construction and respond to surprises during the works. The HOAI accounts for this with two instruments: the refurbishment or modernisation surcharge on the fee and the inclusion of existing building fabric. In the HOAI fee calculator you switch on the surcharge with a tick box.
When is it a conversion?
Under § 2(5) HOAI, conversions are alterations of an existing building involving substantial intervention in its structure or fabric. Modernisations are building measures that lastingly increase its utility value (§ 2(6)). These are distinct from extensions, i.e. additions to an existing building, and from repairs and maintenance. There is no refurbishment surcharge for extensions. For repairs, site supervision can instead be increased by up to 50% (§ 12(2)).
How high can the surcharge be?
The caps for average difficulty are set in § 36 HOAI:
| Service type | Surcharge up to | without agreement |
|---|---|---|
| Buildings | 33% | 20% |
| Interiors | 50% | 20% |
The surcharge must be agreed in text form, taking the level of difficulty into account (§ 6(2)). Without an agreement, a surcharge of 20% is deemed agreed from average difficulty. Since the table values have only been guide values since 2021, higher surcharges for particularly difficult conversions can also be agreed by contract.
Worked example
A house from the 1960s is extensively converted. Eligible costs are €400,000, the project is in zone III and the middle rate is agreed for all work stages. The fee without surcharge is €57,798.50. A 20% refurbishment surcharge adds €11,559.70, raising the fee to €69,358.20 net. With 5% expenses and 19% VAT, the total comes to €86,663.07.
The surcharge applies to every commissioned work stage. In the chart of the HOAI fee calculator you see the surcharge per stage as a separate bar segment.
Existing building fabric: the second lever
Independently of the surcharge, existing fabric can increase the eligible costs. Existing building fabric to be incorporated (mitzuverarbeitende Bausubstanz) is the part of the building that already exists and is technically or aesthetically incorporated through design or supervision (§ 2(7)). Examples are an existing floor slab that has to carry new loads or a facade that is being upgraded for energy efficiency.
Under § 4(3) HOAI, this fabric must be reasonably taken into account in the eligible costs. Its extent and value are determined for the specific building at the time of the cost calculation and agreed in text form. A common approach is to take the replacement value of the affected components, reduce it for age and condition and then apply a share for the degree of incorporation. The result is added to the eligible costs before reading the table value. The calculator does not model this step automatically; simply add the agreed value to the eligible costs.
Surcharge and existing fabric together?
Yes. The two instruments compensate for different kinds of extra effort and can be applied side by side. The surcharge pays for the increased design effort, while the existing fabric reflects that the architect also designs around components whose costs are not incurred again. It is important to record both separately and transparently in the fee agreement.
Tips for clients
Clarify early whether your project counts as a conversion, modernisation, extension or repair. Ask for the surcharge to be justified and agree it in writing, otherwise 20% applies. For the cost side of the conversion, the renovation cost calculator helps. The full calculation is explained in the main article Calculating architect fees in Germany.