The starting case
An office building from the 1980s is to be converted and modernised. The designer is commissioned for work phases 1 to 8.
| Item | Value |
|---|---|
| Building fabric KG 300 from the cost calculation | €2,400,000 net |
| Building services KG 400 from the cost calculation | €900,000 net |
| Existing fabric incorporated, building fabric | €400,000 net |
| Service profile | Buildings |
| Phases commissioned | 1 to 8 |
Step 1: determine chargeable costs
First the other chargeable costs are formed. They comprise essentially the building fabric plus the existing fabric being incorporated.
| Item | Amount |
|---|---|
| Building fabric KG 300 | €2,400,000 |
| plus existing fabric incorporated | €400,000 |
| Other chargeable costs | €2,800,000 |
The threshold rule for technical installations is then applied. They are chargeable in full up to 25 per cent of the other chargeable costs, and at half above that.
| Step | Calculation | Amount |
|---|---|---|
| Threshold | 2,800,000 × 0.25 | €700,000 |
| KG 400 up to the threshold | fully chargeable | €700,000 |
| KG 400 above the threshold | (900,000 − 700,000) × 0.50 | €100,000 |
| Chargeable share of KG 400 | sum | €800,000 |
| Total chargeable costs | 2,800,000 + 800,000 | €3,600,000 |
The effect of the existing fabric is notable. Without applying it, the threshold would be €600,000, the chargeable share of KG 400 €750,000 and total chargeable costs €3,150,000. Applying €400,000 therefore raises the fee basis by €450,000, because it additionally shifts the threshold.
The system is covered in the branch on chargeable costs. It also sets out the threshold rule for technical installations in detail.
Step 2: determine the fee zone
The six assessment criteria must be examined. For a conversion, allocation proceeds by analogy.
| Criterion | Assessment in the example |
|---|---|
| Integration into the surroundings | existing building within the urban fabric, raised requirement |
| Number of functional areas | office with ancillary areas, medium requirement |
| Design requirements | facade retained, interior new, medium requirement |
| Structural requirements | intervention in the structure, raised requirement |
| Building services | complete renewal, raised requirement |
| Fit-out | medium to raised requirement |
Where the assessment yields a consistent picture, the zone is allocated directly. Only where criteria from several zones are applicable and doubt therefore arises is the points method used.
The object list is to be taken into account, but the specific assessment takes precedence. The system is covered in the article on fee zones.
Step 3: read the table value and interpolate
With chargeable costs of €3,600,000 and the zone determined, the band is read from the fee table. The figure falls between two cost steps, so interpolation applies.
Where chargeable costs fall between two cost steps, interpolation between them applies. The calculation is linear and can be followed in four steps.
Fee = lower table value + (upper − lower table value) × (chargeable costs − lower cost step) ÷ (upper − lower cost step)
Interpolation takes place within the same fee zone, not between zones. The result is the fee for all nine work phases.
At this point the decision must also be taken where within the band the fee is agreed. The lower figure is the base fee rate; since 2021 any position within, above or below the band is freely negotiable. The system is covered in the article on fee tables and the base rate.
Step 4: calculate the phases commissioned
Phases 1 to 8 are commissioned, that is all but aftercare. For buildings that gives the following schedule.
| Phase | Percentage |
|---|---|
| 1 Initial appraisal | 2 % |
| 2 Concept design | 7 % |
| 3 Developed design | 15 % |
| 4 Consent design | 3 % |
| 5 Detailed design | 25 % |
| 6 Preparing procurement | 10 % |
| 7 Assisting procurement | 4 % |
| 8 Construction supervision | 32 % |
| Total | 98 % |
The uncommissioned phase 9 carries 2 per cent. 98 per cent of the table value is therefore invoiced.
Step 5: add the conversion supplement
Since this is a conversion, the conversion supplement is added. It acts on the fee determined, that is on the result of step 4.
| Case | Supplement |
|---|---|
| Agreed in text form | up to 33 per cent for buildings |
| No agreement, from an average degree of difficulty | 20 per cent |
| No agreement, below that degree | no supplement |
The third row is frequently overlooked. The fallback does not apply automatically. The system is covered in the article on the conversion supplement.
Step 6: complete the calculation
| Step | Result |
|---|---|
| Chargeable costs | €3,600,000 |
| Fee zone | from the criteria assessment |
| Table value for all phases | from the fee table, interpolated where needed |
| Share of phases 1 to 8 | 98 per cent |
| plus conversion supplement | as agreed |
| plus additional services | under separate agreement |
| plus ancillary expenses | as agreed or on evidence |
| plus value added tax | on the total |
The last three rows are frequently omitted from proposals and then added to invoices. They belong in the same agreement as the fee.
What this example teaches
Three observations hold beyond the individual case. They concern the quantities that move the result most.
The order is not arbitrary. Existing fabric acts before the table, the conversion supplement afterwards. Reversing them gives the wrong answer.
Existing fabric acts twice over. It raises the fee basis directly and additionally shifts the threshold of the special rule for technical installations.
The decisions sit in steps 2 and 3. Zone and position within the band are judgements, not arithmetic. They belong justified and recorded.
Related articles
This article reflects the position of the rules at the date of checking and serves professional orientation. The amounts used are illustrative. It does not constitute legal advice and does not replace assessment of the individual case.