Sector

Engineering firms: destroying project files

25 July 2026 · 9 min read · DeSnipperaar

An engineering firm builds a thick file on every project: technical drawings and calculations, structural reports, tender and quotation documents, confidentiality agreements and the contact details of everyone who worked on it. Part you keep for a long time because of liability, part falls under the tax retention obligation and part contains personal data that falls under the GDPR. This guide shows, by part, what you keep, when it may go and how to destroy it confidentially.

The quick answer: the project file you keep until the end of the liability period, which for construction and structural work can run to ten or twenty years. The project administration falls under the tax seven years. Quotations that did not lead to a commission and personal data without a further ground you clear out earlier. What may go disappears confidentially and with a certificate.

Two frameworks: liability and GDPR

At an engineering firm two things run together. Liability determines how long you may need a file to demonstrate that you calculated and advised correctly. With hidden defects in a structure a claim can surface years after completion, so a complete file is your best defence. Alongside this the GDPR applies, which requires not keeping personal data longer than necessary. Liability sets the floor for what you must keep, the GDPR the ceiling for the personal data you may not keep too long.

So treat the file per type. A structural calculation has a different status than a list of contact persons or an unsuccessful quotation. If you make that distinction, you keep exactly what you must and clear out the rest on time. How long the various parts need to last is set out in how long you should keep documents.

Retention periods by part

The period differs per type of data. The overview below gives the main line. Count the tax period from the end of the financial year and the liability period from the completion of the project.

Part	Starting point	Period
Invoicing and project administration	Tax retention obligation	7 years
Technical drawings and calculations	Liability and limitation	until end of liability
Contract and tender documents	Liability and evidence	until end of liability
Quotations and pitches (unsuccessful)	No retention obligation	shortly after conclusion
Contact persons and client data	GDPR, storage limitation	as briefly as possible
Correspondence and drafts	No retention obligation	clear out at once

Use this as a guideline, not a substitute for your contractual arrangements. When in doubt, consult your legal adviser or insurer. The tax side is in the 7-year tax retention obligation. Related parties that work with the same documents you find at architecture firms with drawings and construction files.

Technical drawings and calculations

Drawings, structural calculations and technical reports form the core of the file. They are the proof that you designed and calculated to the standards, so you keep them for as long as liability runs. As long as a defect can still lead to a claim, a complete and traceable file is your defence. So only clear out once the period has genuinely passed and the document no longer serves any purpose.

Once the period has passed, these documents do not belong in the paper bin. A calculation or drawing on the street reveals how a structure is put together and is competition-sensitive. Large-format drawings and folded working drawings you therefore have destroyed confidentially, just as the construction sector does with inspection reports and blueprints.

Tender and quotation documents

Around a tender you gather calculations, submissions, cost estimates and justifications. For a successful project these documents belong with the project file and you keep them for the liability period. For an unsuccessful quotation the purpose lapses once the tender has concluded and no objection or dispute is in play. You do not keep those documents for years on the chance that they might one day come in handy, because that is not a valid ground.

Quotation documents in particular contain your rates, margins and working method. In the wrong hands that gives a competitor insight into how you calculate. So treat unsuccessful submissions as confidential and clear them out once the purpose has lapsed.

Confidentiality and competition-sensitive data

Many commissions run under a confidentiality agreement. The client expects designs, methods and measurement data to stay in-house, even after the project has concluded. That agreement does not simply end on the day you clear out the file. Anyone who throws a confidential document in with the ordinary waste breaches the agreement and risks a claim and reputational damage. How you handle confidential documents from archive to destruction is set out in destroying confidential documents.

Think of the digital side too. Old laptops, servers and backups hold drawings, models and calculation files. A written-off data carrier with project data deserves confidential destruction just as much as the paper, because it often holds the most complete version.

Contact persons and client data

Every file contains names, roles, telephone numbers and email addresses of clients, subcontractors and colleagues. Those are personal data and fall under the GDPR. You keep them for as long as the project and the liability require, but no longer. Once that purpose lapses you clear out the personal data and keep only what you genuinely still need. That way you avoid managing an outdated address list for which you no longer have a ground.

How to handle it in 6 steps

1. Split the file into administration, technical documents, contract documents and personal data.
2. Determine the liability period per project and count from completion.
3. Clear out unsuccessful quotations once the tender has concluded.
4. Limit personal data to what the project and the liability require.
5. Collect what may go in sealed containers, not in the paper bin.
6. Have it destroyed confidentially with a certificate and record it in your register.

Destroy confidentially with a certificate

Project files are destroyed confidentially, because they contain competition-sensitive, contractual and personal data. The paper and any data carriers travel sealed and stay that way until destruction, so the chain is closed. That closed chain from archive to shredder we describe in chain of custody from archive to shredder.

Afterwards you receive a certificate of destruction with the date, quantity and level. That certificate is your proof towards the GDPR and the client that you acted carefully. Record the destruction in your record of processing. We collect within 20 km of Amsterdam with no call-out charge, work nationwide through pooled collection rounds and charge a fixed price per box or roll container. Drop-off on site is not possible; it works by appointment through collection.

Common mistakes

• Keeping files forever. After the liability period and the tax period the purpose lapses.
• Keeping unsuccessful quotations. Without a commission there is no ground to keep rates and calculations.
• Throwing drawings in the paper bin. They are competition-sensitive and often confidential.
• Forgetting data carriers. Old laptops and backups often hold the most complete project data.
• Keeping no proof. Without a certificate you cannot demonstrate the destruction.

Frequently asked questions

How long does an engineering firm keep a project file?

A project file you keep until the end of the liability period, which for construction and structural work can run to ten or even twenty years. The invoicing and project administration fall under the seven-year tax retention obligation. What no longer serves a purpose after that you clear out confidentially.

May I simply throw away technical drawings and calculations?

No. Drawings, calculations and structural reports are competition-sensitive and often fall under a confidentiality agreement with the client. In the paper bin they end up on the street. Have them destroyed confidentially with a certificate as proof.

What do I do with contact persons and client data in an old file?

Names, roles and contact details of those involved are personal data and fall under the GDPR. Do not keep them longer than necessary for the project and the liability. Once the purpose lapses you clear them out and record it in your record of processing.

How do I destroy a project file in line with the GDPR?

Confidentially and with a certificate of destruction. Paper and data carriers travel sealed and stay that way until destruction, so the chain is closed. You record the destruction in your record of processing.

Conclusion

An engineering firm works with competition-sensitive drawings, structural calculations, contract documents and personal data, between a long liability and the GDPR. Keep the project file until the end of the liability period, keep the administration seven years and clear out unsuccessful quotations earlier. Contact persons and client data you keep no longer than necessary. What may go you have destroyed confidentially with a certificate as proof. That way you meet both frameworks and protect your own knowledge and your clients' data.

Read also: installation companies: destroying customer data, property managers: destroying tenant files, self-storage: destroying customer contracts and the GDPR retention periods cheatsheet.

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Have project files collected? Request a quote via desnipperaar.nl. Within a few minutes you have a fixed price, including a certificate as proof.