What counts as existing fabric
The definition sits in § 2 paragraph 7 HOAI: existing fabric to be incorporated is the part of the object being designed that has already been constructed and is technically or aesthetically incorporated through design or supervision services. The wording is brief and broadly construed.
The term is broadly construed. Three points matter and are routinely overlooked.
Physical alteration is not required. The existing fabric need not be changed for it to count as incorporated. It suffices that the designer engages with it technically or aesthetically.
The boundary lies at mere depiction. Simply drawing existing elements without engaging with them in design terms does not amount to incorporation. Where the depiction serves a design purpose, incorporation is present.
The boundary remains a case-by-case assessment. No sharp line separates incorporation from mere retention of existing fabric. This is precisely why the provision is among the most litigated in the HOAI.
An illustrative case: where the structure of an existing building must be assessed or recalculated in order to decide between conversion and demolition with rebuilding, the cost of that structure will as a rule be taken fully into account if conversion is chosen. The effort arises without any element being renewed.
Why the valuation is open
The regulatory history explains today's imprecision. A comparable provision already existed in the 1996 HOAI. In the 2009 version it was deleted outright and replaced by the option of agreeing an increased conversion supplement instead.
That route proved unworkable. Designers frequently failed to secure the supplement at an appropriate level in contract negotiations, producing uneconomic fees. With the 2013 version the legislator therefore returned to the earlier system, and the 2021 version has retained it.
What did not return was a valuation rule. The legislator confined itself to the indeterminate legal standard of appropriateness.
The formula used in practice
Because the ordinance is silent, a formula has established itself in practice. It is a convention rather than a requirement.
AmvB = M × W × L
| Quantity | Meaning |
|---|---|
| AmvB | chargeable costs arising from the existing fabric |
| M | quantity of existing fabric being incorporated |
| W | value factor of the existing fabric |
| L | service factor for the degree of incorporation |
The value factor is measured by the objective value of the existing fabric in its actual state of preservation. Not by historical construction cost and not by the market value of the property.
The service factor reflects how intensively the design engages with the element in question. An element fully reassessed and integrated into the design carries a different factor from one merely taken into account in its position.
The qualification is decisive: how the individual values are to be determined remains contested and the subject of numerous court proceedings. The formula provides a structure for the argument, not legal certainty. Anyone using it should justify each of the three quantities separately and record the derivation.
Two levels must be kept apart
In the determination, a distinction is drawn between existing fabric of the building fabric and that of the technical installations. The two enter different quantities of the fee calculation.
Existing fabric of the building fabric is applied in full and therefore enters the other chargeable costs. This has a side effect frequently overlooked: it thereby also raises the threshold up to which the cost of technical installations is chargeable in full.
The effect is therefore twofold. A properly determined allowance for existing fabric raises the fee basis directly and additionally shifts the threshold of the special rule for services upwards. The system behind that threshold is covered in the article on chargeable costs.
Establishing the quantity in practice
Factor M in the formula presupposes a condition survey, and one that distinguishes between elements incorporated and elements not incorporated. That distinction is not an incidental task but the substance of the work.
A schedule by element group works well, carrying each element with three entries: quantity, condition and the nature of the design engagement. It is therefore traceable in a dispute.
| Element group | Typical engagement in a conversion |
|---|---|
| Structure | assessment, verification, strengthening where required |
| Foundations | assessment of capacity under changed loads |
| Facade | aesthetic integration, assessment of thermal performance |
| Roof | assessment of condition, capacity and build-up |
| Internal walls | decisions on retention, removal or forming openings |
| Building services | assessment of continued usability and connectability |
Elements that remain unchanged and with which the design does not engage do not belong in this schedule. They are depicted but not incorporated, and applying them would be open to challenge.
The effort for this schedule falls before contract, at a point when no fee entitlement yet exists. That is the practical reason the provision so often goes unused. Economically it is nonetheless the most profitable hour of the project, because it affects the fee basis across every work phase.
The conversion supplement is no substitute
A widespread misconception is to seek to cover the additional effort in existing buildings through a higher conversion supplement rather than by applying the existing fabric. The supplement acts proportionally on a basis that is too small.
The two instruments stand side by side and serve different purposes. The conversion supplement compensates for the increased difficulty and additional effort of working in existing buildings. Applying the existing fabric corrects the fee basis itself.
In practice a missing allowance for existing fabric cannot reliably be offset by a supplement raised a few percentage points, because the supplement acts proportionally on a fee already calculated too low without the fabric. No general statement is possible, however, since the relationship depends on the individual case.
What belongs in the agreement
The HOAI provides for a written agreement on the extent and value of the existing fabric. Where none is reached, the objective value is established by expert assessment and ultimately by the courts.
A clear order of priority follows for practice. It cannot be reversed.
- Establish extent and value before concluding the contract, at least approximately, and attach the derivation as an appendix.
- Show the three factors of the formula separately, so that the reasoning remains traceable in a dispute.
- Record the date of valuation. The state of preservation changes, and with it the value factor.
- Provide for updating. Where the condition survey after contract shows a different extent, adjustment is easier if a procedure was agreed.
The first point carries the greatest economic weight. Anyone who avoids the question of existing fabric at contract stage so as not to jeopardise the commission routinely fails to receive an appropriate fee for the work performed.
On the relationship to earlier minimum-rate case law
Older decisions established that existing fabric is relevant to the minimum rate, raising chargeable costs and therefore the minimum fee. That classification dates from the era of binding price law.
That statement must be read with care in today's framework. Since the 2021 revision the fee tables are no longer binding minimum and maximum rates but values for orientation. The minimum rate as a mandatory floor no longer exists in that form.
The substantive core is unaffected: existing fabric forms part of the basis on which fees are calculated and raises chargeable costs. The entitlement to appropriate consideration continues to follow from § 4 paragraph 3. What has changed is the enforcement mechanism, not the principle.
Related articles
This article reflects the position at the date of checking and serves professional orientation. It does not constitute legal advice and does not replace assessment of the individual case.