Maintaining custom software: how much to budget each year, and for what?

In short

• Two development agencies put the maintenance of custom software at between 15 and 25% of the development price, every year.
• At Kendu Intelligence, you choose a tier (Essential, Standard or Premium) and you propose your monthly payment, at least equal to a minimum stated in your quote.
• After acceptance testing, defects are fixed free of charge for a period stated in your quote. Payment, commitment and termination are also stated there.

Allow 15 to 25% of the development price per year: this is the benchmark given by two development agencies. Let's take an amount chosen for the example: for software invoiced at €40,000, that is €6,000 to €10,000 a year, or about €500 to €833 a month. Depending on the offer, this budget covers hosting, security, fixes and a share of enhancements. Here is how to read an offer, then how we work.

The benchmark: 15 to 25% of the development price per year

Two development agencies publish this benchmark on their websites:

• the first (article of 28 August 2025): “Each year, allow an average of 15 to 20% of the initial cost”. It gives the example of a €200,000 project: €30,000 to €40,000 of maintenance per year. After five years, if the software has stayed healthy, it puts maintenance at around 20 to 25% per year.
• the second (article of 13 April 2026): “Annual maintenance represents 15 to 25% of the initial cost”.

These are estimates from service providers, not an official statistic.

Other providers show monthly packages. Here is what we found on their websites on 13 October 2026:

• a Spanish agency, for websites and web applications: €90, €190 or €390 per month. These packages include 1, 2 or 5 hours of development per month and a reply within 24 h, 8 h or 1 h.
• a French agency, for custom software: hosting and maintenance “from €149/month”.
• another French agency, for the maintenance of automations: “from” €300, €550 or €900 per month, in “no commitment” packages.

None of these three pages says whether VAT is included. Their content also differs: compare what is included, not just the price.

Several things make the bill vary. First, the response time and the hours included: at the Spanish agency, the fastest package costs more than four times the first. Then, according to the first agency cited, “exposure (who uses the software) and dependence on the environment”. In other words: the more your software connects to other services, the more it has to be adapted when those services change.

What maintenance covers: hosting, security, fixes, enhancements

A maintenance offer mixes several things. Check which ones are included.

• Hosting: the servers on which your software runs. The place of installation (with us, on your account with an online host, the “cloud”, or on your servers) is defined with you and stated in your quote. When we host it, it will be in the European Union, and the restoring of backups will be tested regularly. An AI model used for maintenance can be operated outside the EU, under GDPR safeguards.
• Security and updates: software relies on components written by others. Their authors publish patches. We will apply them on planned dates. Each update will first go through your software's automated tests, that is, checks run by a program.
• Fixes: there is a defect when the software does not do what the approved specification provides. The specification, or “spec”, is the document that describes what the software must do. You approved it before development.
• Enhancements: everything that is new. An extra field, an export, a new screen.

Supervision, that is, monitoring your software while it runs, is in preparation. Your software will report the errors it detects by itself: a follow-up request (a “ticket”) will be opened, and we will be alerted. A client area is planned to follow the handling, and a monthly report will say what was done.

Three tiers: Essential, Standard, Premium

The response time is the time we need to start handling your request. It is not the resolution time. The one for each tier is stated in your quote, as is the volume of small enhancements in Premium. In Essential, a defect found after the warranty is fixed separately, on a quote.

How to choose?

• Essential suits a stable tool, used from time to time. It receives security updates.
• Standard suits cases where your team uses it every day: fixes are included.
• Premium suits cases where the tool supports a critical activity, such as scheduling or order taking, or where you want to develop it often.

For example, an SME with 30 employees that schedules its jobs in a custom tool cannot stay blocked for long. For it, a tier that includes fixes (Standard or Premium) is a minimum.

You propose your monthly payment

As with development, you are the one who proposes the price. Each tier has a monthly minimum, calculated from the price of your development and stated in your quote. It is the amount below which we cannot provide the service.

You propose your monthly payment, equal to or above this minimum. And if the minimum is above your budget, a lighter tier may be enough.

The terms of payment, commitment and termination are stated in your quote.

The warranty after acceptance testing

Acceptance testing is the step where you test the delivered software and confirm that it does what the spec provides. From that date, we fix defects free of charge for a period stated in your quote.

The warranty covers gaps against the approved spec. It does not cover new requests: those are enhancements. Nor does it replace maintenance, because it includes neither hosting nor security updates.

We do not offer a full “satisfied or your money back” guarantee: work that has been done cannot be returned. However, the spec you approved serves as the reference. During the warranty, whatever is written in it and does not work is fixed at no cost.

Going further

• Custom software: how you propose your development budget, and what it lets you build.
• Supervision: how your agents and your software will report their own problems to us.
• Data and security: where your data is hosted, and who accesses it.