Capturing knowledge: old projects as a reference library

When experienced people leave a design and engineering practice or a construction company, part of their knowledge leaves with them, namely the part that was never written down. Old projects can capture some of it. Correspondence, meeting minutes, billing records and drawings can be prepared as a searchable reference library in which every hit names the document and the page. Only what has been checked and what you may legally keep belongs in it.

Why the topic is pressing

KOFA study 04/2026 from the Institut der deutschen Wirtschaft (IW) in Cologne draws on special analyses by the Federal Employment Agency (Bundesagentur für Arbeit) and the IAB job vacancy survey. According to it, more than 8.5 million employees subject to social insurance contributions were aged 55 or over between July 2024 and June 2025. That is 25.4 per cent of 33.5 million. In the study’s assessment they will probably retire within the next ten to fifteen years. For the construction sector it gives two examples. Of the specialists in building construction supervision, which include foremen (Poliere) and concrete and building-construction master craftspeople among others, 41.2 per cent are 55 or older. According to the study, more than 1,200 vacancies in this occupation already cannot be filled today, calculated against the available jobseekers. Among the experts in supervision and management in construction planning and supervision (architecture included), the figure is 27.9 per cent of more than 58,000 employees, somewhat above the average across all occupations.

These figures describe the whole economy and two occupations. They say nothing about your practice. Do the count yourself: who will no longer be there in five to ten years, and who alone knows today how a particular project or client works?

The Fraunhofer survey of architecture and engineering practices from March 2026 also lists business succession and the development of young talent among the further strategic priorities that practices named. Knowledge management and knowledge databases are among the topics the respondents called particularly relevant for further training.

What can be drawn from old projects

A reference library answers the question “How did we solve this back then?” Here is an example with invented data: “How was the extra quantity for the earthworks justified and invoiced on the Musterstraße project?” The answer lies in the correspondence, in meeting minutes and in a variation (Nachtrag), and the knowledge base puts the three places side by side. How each hit gets its source reference and how you check it is covered in Why AI answers need a source reference.

1. Old projectsCorrespondence, minutes, billing, drawings
2. Selection and rulesWhat may go in, what may not, when it is deleted
3. Ordering and OCRProject, date, document type, readable scans
4. Search with sourcePer hit: document, page and excerpt
5. Reference librarySpecialists check and maintain the stock

Text version of the graphic

• Layer 1 from the bottom: Old projects, Correspondence, minutes, billing, drawings
• Layer 2 from the bottom: Selection and rules, What may go in, what may not, when it is deleted
• Layer 3 from the bottom: Ordering and OCR, Project, date, document type, readable scans
• Layer 4 from the bottom: Search with source, Per hit: document, page and excerpt
• Layer 5 from the bottom: Reference library, Specialists check and maintain the stock

Material	What can be gained	What to watch for
Correspondence	Agreements, reasons, the course of a point of dispute	Personal data, private matters, individual emails without their context
Meeting minutes	Decisions, responsibilities, reasons for decisions	Draft or approved version, date
Billing (invoices, site measurements, variations)	How variations were justified and invoiced	Price level and scope of works at the time, never adopt figures unchecked
Drawings	Drawing revisions, component designations, revision history	Scan quality, read dimensions only from the original

OCR (text recognition) can make content accessible from scans that nobody can find today. How well that works depends on the quality of the originals, and a sample check shows it (more in Old plans and poor scans).

What does not suit

• Unchecked figures as benchmark figures: Prices and quantities from old projects describe the project of the time. Deriving benchmark figures for new projects from them requires the price level, the scope of works and adjustment for special cases. That is a separate piece of work with its own check.
• Personal data without a purpose: The GDPR requires data that is adequate for the purpose and limited to what is necessary. It may be stored so that people are identifiable only for as long as the purpose requires (Article 5(1)(c) and (e)). Personnel files, tenant files, health data and private emails do not belong in a general project search.
• Drafts and superseded versions: They can only be included if they are clearly marked. Otherwise the search quotes the wrong version.
• Knowledge that is written down nowhere: Why a detail was solved in a particular way is rarely in the document. For that you need a conversation with the people who are leaving, while they are still there. The knowledge base gives that conversation a starting point: you can put questions about a concrete document instead of asking in general what the successor should know.

Retention, deletion and limitation in general terms

A reference library is an additional place where data is stored. That is why the rules on retention belong with it. The following is for orientation only (as of November 2026).

Section 147(3) of the German Fiscal Code (Abgabenordnung, AO) names ten years for books, records and annual accounts, eight years for accounting vouchers and six years for the other documents listed there. These include commercial or business letters received and copies of those sent. Under subsection 4 the period starts at the end of the calendar year in which, for example, the letter was received or sent. Under subsection 3, sentence 5, it does not expire while the documents are still relevant for taxes whose assessment period is still running.

For defect claims on a building, section 634a(1) no. 2 of the German Civil Code (Bürgerliches Gesetzbuch, BGB) provides a limitation period of five years. The statute covers planning and supervision services there too. The period starts with acceptance (subsection 2). If the contractor fraudulently concealed a defect, the regular limitation period applies, but not before the five years have run (subsection 3). If the VOB/B (the German standard contract terms for construction works) has been agreed and the contract sets no period, section 13(4) no. 1 VOB/B gives only four years for buildings. Which period applies to your contract is stated in the contract.

An example with invented data shows how far apart the periods lie. An acceptance, and the receipt of an invoice and a letter, fall on the same day in March 2026.

1. 15.03.2026 Acceptance, invoice and letter Example with invented data: all on the same day.
2. March 2031 Defect claims time-barred Five years from acceptance (section 634a BGB). A term in the contract can differ.
3. 31.12.2032 Business letter: six years Letter received (section 147 AO). The period starts at the end of 2026.
4. 31.12.2034 Invoice: eight years Accounting voucher (section 147 AO). The period starts at the end of 2026.
5. 31.12.2037 Annual accounts: ten years Example: drawn up in 2027, period from the end of 2027.

Text version of the graphic

• 15.03.2026: Acceptance, invoice and letter. Example with invented data: all on the same day.
• March 2031: Defect claims time-barred. Five years from acceptance (section 634a BGB). A term in the contract can differ.
• 31.12.2032: Business letter: six years. Letter received (section 147 AO). The period starts at the end of 2026.
• 31.12.2034: Invoice: eight years. Accounting voucher (section 147 AO). The period starts at the end of 2026.
• 31.12.2037: Annual accounts: ten years. Example: drawn up in 2027, period from the end of 2027.

The timeline shows that the periods do not end together and do not start at the same point. For limitation, acceptance counts. For retention, the end of the year counts. The periods can also be extended, for example under section 147(3), sentence 5 AO. The GDPR additionally requires that personal data is not stored in identifiable form for longer than the purpose demands.

The rules serve different purposes and do not say the same thing: the Fiscal Code requires retention, the GDPR limits storage, and limitation only determines how long claims can be enforced. Which period applies to which record, and when deletion is required, is decided case by case by your tax adviser (Steuerberater), your data protection officer and your lawyer. Technically it should be settled that a deleted file also disappears from the working copy, the index and the history of the knowledge base.

Who maintains the reference library

A reference library goes out of date if nobody looks after it. So name one person who takes in new projects when they close, checks the ordering and records the deletion date of the file at the time of intake. The conversations with people who are leaving need a fixed date before they go, ideally along a gap list: which questions stayed without a hit in the search, and who can answer them? You write the answers down and file them with the projects. They then become part of the reference library too, and they carry a date and a name as their source reference.

How to proceed

First choose one completed project and agree with the managing director which folders may go in. Then order the records by project, date and document type (suggestions in Project folder structure and file names) and have the scans made readable. A named contact person checks the hits against test questions from everyday work. As an assumption, four to six weeks make sense for a pilot at the start. Questions without a hit end up as a gap list with this person, and the list decides what has to be written down or secured in conversation.

What you can do now

1. List who might leave in the next five to ten years and which projects or clients only that person knows.
2. Choose a completed project and note five questions a successor would ask about it.
3. Clarify with your tax adviser and data protection officer which records you keep for how long and when they are deleted.
4. Decide which folders never belong in a shared reference library.