An HOAI calculator returns a number. How it got there often stays unclear. That is exactly what becomes expensive as soon as the fee zone, the fee rate or the phase percentages are set incorrectly.
Yet only three figures determine your fee: the eligible costs, the fee zone and the service phases you have been commissioned to deliver. Anyone who derives these three figures properly can recalculate any HOAI fee themselves and spot errors in someone else's calculation immediately. Here is how to do it.
The key points at a glance:
- The fee is the table value (eligible costs plus fee zone) multiplied by the percentages of the commissioned service phases
- For most service profiles, the HOAI distinguishes five fee zones and up to nine service phases
- Without a written fee agreement, the minimum rate (base fee rate) applies automatically
- A fee zone classified too conservatively costs several thousand euros per project at €2 million in eligible costs
How do you calculate the HOAI fee in 3 steps?
The calculation path is the same for every service profile: determine the eligible costs, read the table value against the fee zone, apply the service phase percentages. Everything else is detail within these three steps.
Formula: Fee = table value (from eligible costs, fee zone and fee rate) × sum of the commissioned service phase percentages
Step 1: determine the eligible costs
Every HOAI calculation starts with the eligible costs. For buildings and interiors under Section 33 HOAI, these are the construction costs under DIN 276, limited to the eligible cost groups.
Other service profiles work with different input values: areas in hectares or accounting units. Which of the 22 HOAI service profiles applies therefore also determines which value you use in step 1.
Step 2: set the fee zone and read the table value
In the fee table, the eligible costs meet the fee zone. That combination produces the fee range between the minimum and maximum rate. If your cost figure falls between two reference points in the table, interpolate linearly.
The fee rate within that range is a matter of negotiation. It must be agreed in writing. Without a written agreement, the minimum rate applies. The HOAI 2021 sets out this fallback explicitly.
Step 3: apply the service phase percentages
The HOAI distributes the total fee across up to nine service phases (LP 1 to LP 9). If the client commissions only part of them, add up the percentages of the commissioned phases. Multiply that sum by the table fee from step 2. Partial services within a phase reduce the respective percentage further.
Worked example: office building with €2 million in eligible costs
Here is the calculation path with concrete figures:
- Eligible costs: €2,000,000 in construction costs under DIN 276
- Fee zone and rate: fee zone III (average planning requirements), minimum rate. Assumed table value: €250,000 base fee for the full service profile.
- Service phases: LP 1 to LP 8 are commissioned, together 98 % of the fee total. Result: €250,000 × 0.98 = €245,000.
If the client takes on parts of the work and commissions only LP 1 to LP 4 (27 %), the fee falls to €67,500. The calculation path is identical for engineering structures under Section 44 HOAI. Only the input value, the table and the phase percentages change.
Which fee zone applies to your project?
For most service profiles the HOAI distinguishes five fee zones. They reflect the difficulty of the planning task: zone I stands for simple requirements, zone V for exceptionally difficult planning tasks. Classification follows a defined procedure based on evaluation points.
Evaluation points instead of gut feeling
For buildings and interiors, the requirements relating to construction, technical equipment, design, fit-out and diversity of use all feed into the assessment. Each characteristic contributes points. The total assigns the property to a fee zone.
In practice this classification is often done intuitively, because the points logic looks time-consuming. The result: projects end up in zone II or III even though their characteristics would justify zone III or IV. At €2 million in eligible costs, the difference between zone III and IV can amount to several thousand euros. Across 20 to 30 live projects, that adds up to five-figure sums per year.
Classifying too aggressively is just as risky, however. Clients contest inflated assessments during the project, and then the entire fee basis is up for debate.
When a reassessment pays off
Many firms set the fee zone once at the start of a project and never review it again. If the scope of the property shifts through change orders, a reassessment of the zone may be justified. So review the classification again with every significant planning change.
How is the fee distributed across the service phases?
The HOAI assigns each service phase a percentage share of the total fee. For buildings and interiors, this distribution applies:
Source: Section 34 (3) HOAI.
Accounting for partial services correctly
If individual service phases are only partly delivered or replaced by the client's own work, the percentages must be adjusted. That sounds manageable. In everyday project work it quickly becomes confusing as soon as several service profiles run in parallel and clients take on partial services themselves.
An example from practice: an engineering firm plans a bridge with €3.5 million in eligible costs under Section 44 HOAI, fee zone III. LP 1 to LP 6 are commissioned, with LP 5 agreed at a reduced percentage because the client takes on parts of the detailed design. Every one of these adjustments must be documented in the quote, otherwise it is barely enforceable later.
Which mistakes cost you fee income under the HOAI?
The calculation logic is regulated. Fee income is lost anyway, and almost always at the same five points.
No written fee agreement
The HOAI 2021 came into force in response to the 2019 ruling of the European Court of Justice. It retains minimum and maximum rates as the standard case but explicitly permits fees agreed in writing that deviate from them. The flip side: anyone who agrees nothing in writing falls back automatically to the minimum rate. The relaxed maximum rate opens up room upwards that only firms with a clean calculation basis can use.
Fee zone set once, never reviewed
The second classic: a conservative zone classification out of habit, combined with no reassessment when planning changes. Together, the two create the structural fee deficit that only becomes visible in the year-on-year comparison.
Service phases without evidence
Service phases that were delivered but never properly documented are hard to enforce in a dispute. This applies particularly to LP 5 and LP 6, which are frequently lumped together or made a flat fee during negotiations. Anyone who has to prove which phase percentages were actually delivered needs complete time records per service phase. We have written separately about how to bill fee phases and milestones cleanly.
Change orders delivered but never invoiced
If the client commissions planning changes beyond the agreed service profile, additional fee claims arise. In practice these extra services are often delivered and never charged, because nobody reconciles the hours worked against the agreed scope. Systematic change order management in engineering firms turns those invisible hours into defensible supplementary quotes.
Broken handovers between calculator, Excel and invoicing
The external HOAI calculator produces a value that is copied by hand into the quote and from there into separate invoicing software. Every transfer is its own source of error: typos, version conflicts, lost traceability. If the client asks six months later how the fee for LP 3 was composed, somebody has to reconstruct the quote calculation manually.
HOAI calculation in daily practice: where Excel fails
With three projects, the combination of an external calculator and a spreadsheet works. With 20 parallel engagements, each with its own service profile, fee zone and phase mix, it becomes an administrative burden.
Standardising downwards
Under time pressure, firms standardise their calculations downwards. Fee zones are set conservatively to avoid discussions. Service phase structures are simplified to produce quotes faster. Both depress the achievable fee, and the loss stays invisible in day-to-day business.
On top of that come the structural Excel problems: no version control, manual updates when fee tables change, and no connection between a quote item and a recorded hour. Beyond a certain number of projects, more diligence alone no longer solves this.
Calculation with no link to controlling
The most expensive break sits between calculation and control. Firms can see how many hours a live project has consumed. What they rarely see is how those hours relate to the calculated phase percentage. That connection is exactly what makes project controlling in engineering firms actionable: if you know how much fee was agreed for LP 4 and how many hours are actually flowing into it, you spot deviations early and calculate follow-up projects more precisely.
The moment firms make the switch
The pressure to act rarely comes from theory. It comes from a fee dispute that cannot be resolved with the available documents. From annual accounts in which hours worked and amounts invoiced drift apart permanently. Or from a growth step that overwhelms the Excel processes. In those moments, a system that connects calculation, service records and billing becomes an economic necessity.
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The HOAI calculator in ZEP: costing inside the quote
ZEP Professional builds the HOAI calculator directly into quote creation. When you create a quote, the entry "HOAI Honorarberechnung" is available and opens a form with a dedicated HOAI tab.
Service profile, zone and rate in one form
You select the service profile from all 22 HOAI service profiles, for example Section 35 buildings, Section 40 outdoor facilities or Section 44 engineering structures, including consulting services from Annex 1. Your selection automatically controls the relevant input value: eligible costs, area in hectares or accounting units.
You set the fee zone and fee rate directly in the form and activate service phases via checkbox. The HOAI standard percentages are pre-filled and can be adjusted for partial services. All 20 fee tables are stored in the system, with 3,090 individual values checked against the official legal text. ZEP calculates intermediate values by linear interpolation, and the amounts appear live.
Quote items without transfer errors
On saving, ZEP automatically generates structured quote items per service phase, numbered hierarchically. Phases that were not commissioned remain as inactive items and can be activated later without feeding into totals or documents. The service profile, fee zone and fee rate are documented in the quote's comment field. In a fee review, the derivation is therefore traceable at any time.
From costing into project controlling
The agreed service phases and fee amounts pass straight into project controlling. With project time tracking, your staff book hours directly against service phases. That way you see continuously whether a phase is running outside its calculated frame, and you have the data basis for change orders. An external HOAI calculator gives you a number. ZEP gives you the connected data basis from quote to invoice.
Conclusion: three steps that secure fee income immediately
The HOAI sets the framework. What your firm invoices within that framework depends on the precision of your costing. Three concrete steps for this week:
Review the fee zones of your live projects. Apply the evaluation points logic consistently, including on projects that habitually run in zone II or III. A properly justified reassessment increases your fee claim defensibly.
Put fee agreements in writing. Check which projects only have verbal or incomplete agreements. In case of doubt, the minimum rate applies there.
Connect costing and time tracking per service phase. Every delivered phase needs time records and documentary evidence. That protects you in fee disputes and supplies the figures for the next, more precise calculation.
FAQs
What are eligible costs under the HOAI?
Eligible costs are the calculation value on which the HOAI fee table is based. For buildings and interiors under Section 35 HOAI, these are the construction costs under DIN 276, limited to the eligible cost groups. Outdoor facilities and engineering structures have their own definitions. Any deviation in how you draw the boundary shifts the entire fee calculation.
How do I determine the fee zone under the HOAI?
The fee zone is determined using evaluation points. For buildings under Section 35 HOAI, the requirements relating to construction, technical equipment, design, fit-out and diversity of use all feed in. Each characteristic contributes points, and the total assigns the property to one of the five zones. Classifying too low leads to a structural loss of fee income.
What applies if no fee was agreed in writing?
The minimum rate applies. The HOAI 2021 permits fee agreements that deviate from the standard rates, but requires them to be in writing. Without a written agreement, the fee claim falls back automatically to the minimum rate, even if the work delivered would justify a higher fee.
How do I calculate intermediate values in the fee table?
By linear interpolation. If the eligible costs fall between two reference points in the fee table, the fee is calculated proportionally between the two table values. This method is part of the HOAI system and applies to all fee tables.
What changed with the HOAI 2021?
The HOAI 2021 responds to the 2019 ruling of the European Court of Justice, which held the binding minimum and maximum rates of the previous version to be incompatible with EU law. The new version retains minimum and maximum rates as the standard case but permits deviating fees agreed in writing. It applies to contracts concluded from 1 January 2021.
Can I calculate HOAI fees directly in my project software?
Yes. ZEP Professional integrates an HOAI calculator into the quoting module: select the service profile from all 22 HOAI service profiles, set the fee zone and fee rate, and activate service phases via checkbox. All 20 fee tables with 3,090 verified individual values are stored in the system; amounts appear live and are carried over as quote items.
