The precondition: conversion or modernisation
The supplement presupposes that the project qualifies as an Umbau, conversion, or Modernisierung, modernisation, within the HOAI definitions. What the contract calls the project does not settle that.
| Term | Definition | Distinguishing feature |
|---|---|---|
| Conversions | alterations to an existing object involving substantial intervention in its structure or fabric | the substantial intervention |
| Modernisations | building measures durably increasing the utility value of an object | insofar as not an extension, conversion or repair |
| Repairs | not treated as work in existing stock | separate provision through construction supervision |
Both count as work in existing buildings. Pure repair and maintenance are not covered; for those the HOAI provides separately, allowing agreement in text form that the percentage for construction supervision be increased by up to 50 per cent of the weighting of that phase.
The level of the supplement
| Type of object | Maximum agreeable in text form |
|---|---|
| Buildings | up to 33 per cent |
| Interiors within buildings | up to 50 per cent |
The rates each relate to an average degree of difficulty and are added to the fee determined. For open spaces the provision applies correspondingly.
Without an agreement in text form, a supplement of 20 per cent applies as agreed, and then only from an average degree of difficulty upwards. The second half of that sentence limits the first considerably.
That final clause is decisive and frequently omitted. The fallback rate does not apply automatically to every project in existing buildings but only from an average degree of difficulty, corresponding in practice to fee zone III. On projects falling into a lower fee zone, no supplement arises without an agreement.
In practice it follows that the supplement belongs in text form, even where 20 per cent would be acceptable. The fallback is a safety net, not a rule.
The supplement does not replace the existing fabric
This is the most important point of this article and at the same time a widespread misconception. It costs more in the outcome than any argument about the level of the supplement.
Alongside the conversion supplement stands the existing fabric being incorporated, which must be appropriately taken into account within chargeable costs. Both instruments operate side by side and do not exclude one another. The supplement provision applies expressly even where the existing fabric has been taken into account within chargeable costs.
They also act at different points in the calculation. The order in which they bite decides the result.
| Instrument | Acts on | When it is fixed |
|---|---|---|
| Existing fabric incorporated | chargeable costs, that is the fee basis | before contract, with extent and value |
| Conversion supplement | the fee determined, that is the result | in text form, reflecting the degree of difficulty |
| Fee zone | the classification within the fee table | using the assessment criteria |
A common error is to forgo applying the existing fabric and seek to cover the additional effort through a higher supplement alone. That does not work reliably in arithmetic terms, because the supplement acts proportionally on a fee already resting on too small a basis without the fabric.
The system of existing fabric is covered in the article on existing fabric in chargeable costs within the cost planning subject. It also sets out how its extent and its value are derived.
The fee zone in existing buildings
A third instrument is regularly overlooked. For conversions and modernisations the fee zone must be determined separately, namely the zone to which the project is allocated applying the assessment criteria by analogy.
That is no formality. Projects in existing buildings frequently place higher demands on integration, design and building services than comparable new builds and therefore fall into a higher fee zone.
The zone acts before the supplement: it determines which band of the fee table is relevant at all. The system is covered in the article on fee zones.
The supplement is not a minimum rate
One point concerns the placing of older case law. Earlier decisions established that the conversion supplement has no minimum-rate character.
Under the former price law that carried a concrete meaning: the supplement could not be claimed on the argument that it formed part of a mandatory minimum rate. Since binding price law fell away, the question is moot in that form, because no mandatory rates exist.
What remains in practice is the fallback: without an agreement, 20 per cent from an average degree of difficulty applies as agreed. That fiction operates whether or not the parties discussed the supplement.
A worked example of how they interact
The three instruments act in sequence and at different points. A simplified example shows the order.
| Step | Effect |
|---|---|
| Chargeable costs from the cost calculation | starting quantity |
| plus existing fabric incorporated | raises the fee basis |
| Classification into the fee zone | determines the relevant band of the table |
| Reading the fee from the table | yields the fee determined |
| plus conversion supplement | raises the result proportionally |
The order is not arbitrary. Applying the existing fabric acts before the table is read; the supplement acts afterwards. Both raise the fee, but one through the basis and the other through the result.
It follows too why forgoing the existing fabric cannot be offset by a higher supplement: the supplement acts proportionally, and a percentage of too small a basis stays too small. The supplement is also capped in amount, while applying the existing fabric is not.
A further point concerns the special rule for technical installations. Existing fabric of the building fabric enters the other chargeable costs and thereby also raises the threshold up to which the cost of technical installations is chargeable in full. Applying it therefore acts twice over, as described in the article on chargeable costs.
What belongs in the agreement
Four entries belong in the agreement, and all four settled before contract. None of them can be imposed unilaterally afterwards.
- The classification of the project as conversion, modernisation, repair or new build, since which provision applies depends on it.
- The extent and value of the existing fabric incorporated, at least approximately, with a recorded derivation.
- The fee zone, applying the assessment criteria by analogy to the existing building.
- The supplement in text form, with reasons based on the degree of difficulty.
The first governs the other three and is the most frequently skipped. A project the client calls a refurbishment may legally be a conversion, a modernisation or a repair, each with different consequences for the fee.
Related articles
This article reflects the position of the rules and case law at the date of checking and serves professional orientation. It does not constitute legal advice and does not replace assessment of the individual case.