Home / Custom ERP extensions

Build beside your ERP

Custom ERP extensions, without replacing your ERP

For organisations of 250 to 5,000 staff, a fully custom-built ERP is rarely the right call: you would carry every legal change and upgrade yourself. Keep the standard ERP clean and build custom work beside it through its APIs.

For IT managers, information managers and CIOs closing a gap in their ERP: the routes compared, what AFAS, Exact, SAP Business One, Business Central and Odoo expose to custom software, and who does what once a third supplier joins your ERP partner and ICT partner.

By Tom Joseph · Last updated: 29 October 2026

Free consultation Pricing

In short

What sets your organisation apart from a competitor is rarely the general ledger, the VAT return or the payslip. It is how you quote, plan, deliver and service. That part earns custom software; the rest is better off in a standard package the vendor keeps current. So when we build custom ERP extensions, the ERP stays standard and remains the source of truth, and whatever it cannot do we build beside it as a module, app, portal or integration. This is part of our custom software development service.

The trade-off

Why a fully custom ERP rarely fits

For an organisation of 250 to 5,000 staff the sums usually work the other way. You are not buying the first version only, you are buying every version after it: each change to VAT rules, payroll tax and reporting requirements, each security update and each new link to a bank or webshop becomes your own project. In a standard package the vendor pays for that work and spreads it across all its customers.

Then there is the risk an IT manager fears most: owning something nobody else can support. A home-grown ERP depends on what a handful of developers know, and your accountant and auditor have never seen it.

So the question is rarely whether you need custom software, but where it belongs: inside the ERP or beside it. For the broader choice between a package and custom software, read custom or off-the-shelf software.

Four routes

Replace, modify inside the package, or build beside it

Route	Fits when	What to watch
Move to another standard ERP	The package no longer covers your core processes or the vendor is retiring your version.	A migration hits finance, logistics and HR at once. Plan for a programme, not a project.
Modify inside the ERP	The package has an extension model, such as AL in Business Central or Odoo modules, and the change belongs in the posting process.	Have whoever runs the upgrades build it, usually your ERP partner.
Build beside it	The work is something the ERP does not need to know about: a portal, a field-service app, document processing or AI on ERP data.	Use published APIs only, write back as ordinary transactions and keep the logic outside the package.
Fully custom ERP	Your core process exists in no package and is your entire business model.	Rare at 250 to 5,000 staff. From then on you carry every legal change yourself.

Clean core

Clean core: keep the standard clean, build beside it

SAP calls the principle clean core: extensions stay strictly separate from the standard software, the policy is zero modifications, and custom code talks only through upgrade-stable interfaces (SAP Learning). The term comes from the S/4HANA world, but the lesson holds for any ERP: whatever you change in the core, you retest and sometimes rebuild at every upgrade.

Microsoft built Business Central on the same idea. An extension defines functionality as an addition to existing objects rather than changing the base code (Microsoft Learn).

For custom work beside your ERP, clean core comes down to three agreements: no direct database edits that bypass the API, no logic that only works with one version of the package, and the ERP as the single source of master data.

What we build

Modules, apps, portals and integrations around your ERP

Custom modules beside the ERP. A costing tool, planning board or returns process with its own screens, taking items, customers and orders from the ERP and writing results back as ordinary transactions.
Apps for people who do not sit at the ERP. Engineers, drivers and warehouse staff get an app showing only what they need, signed in through your existing identity management.
Portals for customers, dealers and suppliers. Order status, packing slips, invoices and serial numbers from the ERP, without outsiders needing an ERP licence.
Integrations. ERP to webshop, WMS, TMS, CRM or ticketing system, with error handling, logging and an owner. See API integrations for what breaks after go-live.
AI on ERP data. Reading documents, flagging exceptions, preparing drafts. The ERP stays the place where a person approves.

By package

What each ERP package exposes to custom software

ERP	Way in for custom work	What to know first
AFAS Profit	REST or SOAP API: GetConnectors read data, UpdateConnectors insert, update or delete it (AFAS).	An App Connector sets which connectors a token may use: your authorisation model.
Exact Online	REST API per administration (division), with webhooks on part of the endpoints (Exact).	API limits apply. Design around webhooks and filtered queries, not fetching everything every minute.
SAP Business One	Service Layer: a REST API following OData, versions 3.0 and 4.0 (SAP).	Your version and add-ons decide which objects and fields you see.
Dynamics 365 Business Central	API v2.0, enabled by default online, plus AL extensions for changes inside the package (Microsoft).	Standard APIs cannot be extended with fields; that needs a custom API. Too many requests return HTTP 429 and a request may run ten minutes (limits).
Odoo	Since version 19 the JSON-2 API with API keys; XML-RPC and JSON-RPC are removed in Odoo 22 and Odoo Online 21.1 (Odoo).	The external API is only available on a Custom plan, not on One App Free or Standard.

AI on ERP data

AI on your ERP data, without changing the ERP

AI is exactly the kind of custom work that belongs outside the ERP. A model that reads purchase invoices or estimates lead times changes faster than your ERP's release cycle, so keep it in a layer of its own.

We start read-only: the AI reads orders, stock and invoices, and delivers signals and proposals without changing anything. Only when that proves right during a shadow period does the layer write back, and then as a draft through the same API a user would use: a draft order, a coded purchase invoice, a planning proposal. A colleague approves it in the ERP screens they already know, so the audit trail stays in the ERP.

The patterns are worked out in AI with your existing ERP; for the accounting packages see connecting AI to Exact Online and AFAS.

Examples

Four worked examples, by sector

Wholesaler on Exact Online (example). A dealer portal shows order status, packing slips and invoices from Exact, and an assistant answers lead-time questions from the same data. The portal only reads.
Installation company on AFAS Profit (example). Engineers fill in the work order (werkbon) in an app; hours and materials go to AFAS as a draft through an UpdateConnector, where the project lead releases them.
Manufacturer on Business Central (example). For a quote request with a drawing, a costing assistant proposes items and operations from the bills of materials and routings in Business Central. The estimator adjusts and enters the quote in the ERP.
Logistics provider on Odoo (example). Incoming CMRs and packing slips are read and proposed as receipts in Odoo through the JSON-2 API; quantity differences go to the team lead.
These are examples of the pattern, not client cases.

Who does what

Who does what beside your ERP partner and ICT partner

Party	Responsible for	Not for
Your ERP partner	Configuration, licences and upgrades of the package, and extensions that genuinely belong inside the ERP. Reviews the custom design against the APIs used.	Apps, portals and AI layers outside the package, unless you agree otherwise.
Your ICT partner	Identity and access, network, workplaces and the environment the custom work runs in, with monitoring and backup under your existing agreements.	How the custom work behaves functionally.
Your IT department	Ownership and priorities, the change through your change process, and acceptance in the test environment (DTAP).	The build itself.
Crux Digits	Design, build and testing of the module, app, portal, integration or AI layer; documentation, runbook and handover of all source code.	ICT management, hosting, helpdesk or configuring your ERP.

Cost and timeline

What custom work beside your ERP costs and how long it takes

Step	What you get	Duration	Price excl. VAT
AI Audit & Strategy	Your ERP landscape, APIs, data quality and processes reviewed; one scoped piece of custom work with a fixed price.	1–2 weeks	€2,500
Production-ready MVP	The module, app or integration working on your own ERP data in a test environment, tested with the people who will use it.	4–6 weeks	€20,000
Production	Robustly connected to your ERP and other systems, with roles, monitoring, documentation and handover to your IT department and ICT partner.	8–12 weeks	from €50,000
What you always keep	All source code, documentation and the test set, and one named engineer as your contact. Move on to the MVP within 60 days and the audit fee is credited in full. See pricing.

FAQ

Frequently asked questions

What is a custom ERP?

A custom ERP is an ERP system shaped around one organisation's processes: either built entirely from scratch, or a standard package with custom extensions around it. At 250 to 5,000 staff the second is almost always wiser: the package handles accounting, VAT and payroll, and the custom work covers what sets you apart, such as quoting, planning or service.

Can you extend our ERP without replacing it?

Yes, that is exactly what we do. We build modules, apps, portals, integrations and AI applications beside AFAS, Exact, SAP Business One, Business Central or Odoo, through the APIs the package publishes. The ERP remains the source of master data and the place where postings are approved, so an upgrade of the package does not break your custom work.

What do custom ERP extensions cost?

It starts with a €2,500 audit over 1–2 weeks that reviews your landscape and sets a fixed price for the first build. A Production-ready MVP costs €20,000 over 4–6 weeks; production starts from €50,000 over 8–12 weeks. All prices exclude VAT, and if you move on to the MVP within 60 days the audit fee is credited in full.

How do we keep our ERP partner involved?

By having them at the table from the audit onwards. Custom work that reads and writes only through published APIs leaves the package itself untouched. We record which connectors or APIs we use and with which rights, and have your ERP partner review the design before the build starts.

Who runs the custom work after delivery?

Your own IT organisation, together with the ICT partner you already have. We hand over all source code, documentation, a runbook and the test set, and pass the environment to whoever manages it. We do not provide ICT management, hosting or a helpdesk. For AI systems we built, a support arrangement with us is available.

Does it matter whether we run AFAS, Exact, Business Central or Odoo?

Yes, mostly for the way in and the limits. With AFAS and Exact Online custom work goes through their APIs; Business Central and Odoo also allow extensions inside the package. Odoo opens its external API only on a Custom plan, and Business Central temporarily refuses requests with HTTP 429 when an integration asks too much. We map those limits during the audit.

Keep your ERP and build what is missing

A free 30-minute call: you describe the gap in your ERP and your current landscape, we tell you whether it belongs beside the package and what the first step costs.

Book a free consultation →