Custom ERP vs off-the-shelf: build the operations, buy the accounting

If your buying, selling, stock and invoicing look like your industry’s, buy a packaged ERP. If the way you work is why clients choose you, build that part, and keep the accounting packaged.

We are an engineering firm that builds custom operations software, so this guide also says when to buy instead of hiring a firm like ours. Most companies of 10 to 500 people end up with both: packaged accounting, plus a custom operations layer synced to it.

The short answer

An ERP is one system for orders, stock, purchasing and accounting. Most small and mid-size firms run something smaller: accounting software such as Xero, QuickBooks or Sage, with spreadsheets, email and WhatsApp around it. That is the real starting point, and the question it raises is what to buy, what to build, and how the two fit.

Term ERP Enterprise resource planning: the central software that runs orders, stock, purchasing and accounting in one place, such as SAP, Odoo or Microsoft Dynamics.

Buy when your processes look like your industry’s

If you quote, order, ship and invoice the way most firms in your trade do, a packaged ERP such as Odoo, Microsoft Dynamics 365 Business Central, NetSuite or Sage will fit with configuration, and tools such as Odoo Studio add fields and approval rules without code. Buy it, train the team, and stop reading here. It is the simplest path, and plenty of firms should take it.

Build when the way you work is why clients choose you

Some firms win on how they work: a pricing model nobody else offers, an approval chain clients trust, a field process that is faster than the competition’s. Bend that into a packaged ERP and you either lose the edge or pay for customisation that every upgrade puts at risk. That part is worth building.

For most firms, both: packaged accounting plus a custom operations layer

The ledger, tax and payroll look the same in every company, so buy them. The daily work, the approvals, the portals and the live numbers are where firms differ, so build those, and connect them to the accounting software. It is the answer for most firms, and the one the rest of this guide unpacks.

Custom vs off-the-shelf ERP, side by side

Three options, in words, since prices depend on your team and your vendor.

Three ways to run your back office, compared

	Off-the-shelf ERP	Heavily customised off-the-shelf	Packaged accounting + custom operations layer
Time to first use	Weeks, for standard modules	Months: each customisation is specified, built and tested	Weeks for the first operations app; accounting stays as it is
Fit to your process	The team adapts to the software	Close, through configuration and custom code	Operations follow how the team works; accounting follows the standard
Who changes it, and how fast	The vendor, on its roadmap; you configure what it allows	Your integrator, one change request at a time	Engineers you choose, in short rounds; the vendor keeps the ledger current
Upgrade risk	Low on standard modules: the vendor upgrades them	High: every upgrade means re-testing each customisation	Low on accounting; the custom layer moves on your schedule, with tests
Per-user cost as you grow	Grows with every person who needs a licence	Grows with users, plus the integrator's time	Licences for the finance team; the custom app adds hosting, in your own account
Who owns code and data	You run the vendor's code; your data comes out through its export tools	Your data; the customisations, as your contract says	You own the custom code, its accounts and its data from day one
If the partner leaves	The vendor stays; another partner takes over the setup	The customisations depend on whoever knows them	It keeps running in your accounts; the next engineer reads the decisions and takes over
Accounting and tax compliance	The vendor keeps it current	The vendor keeps the core current	The accounting vendor keeps it current
Best for	Processes that look like your industry's	Firms committed to one ERP, with a few gaps it cannot fill	Firms whose way of working is why clients choose them

A fourth option is a fully custom ERP, ledger included. We rarely advise it: you would then keep up with every tax and accounting rule yourself, for a part that gives you no edge.

Signs your packaged ERP no longer fits

You may already own an ERP. These are the signs the operations side has outgrown it.

The team keeps a parallel spreadsheet

The order is in the ERP, but the week’s schedule or the margin on each job is worked out in a spreadsheet someone updates every morning, then typed back in. For example, a wholesaler whose stock figure differs between the warehouse sheet and the accounting software: every copy is a chance for two numbers to drift apart, and for a decision to rest on the wrong one.

The list of customisations is longer than the list of standard features you use

Each customisation made sense when it was added. Together they make every upgrade a project: each one is checked, fixed and tested again before the team can move to the new version. Odoo’s own upgrade guide says a database with custom modules cannot be upgraded until those modules are ready for the new version, and Microsoft asks Business Central customers to keep testing their extensions against each update.

Per-user fees grow faster than the value

Field and warehouse staff often need one screen: today’s jobs, a photo, a signature. When each of them needs a full licence to reach that screen, the bill grows with the team while the value stays flat. Check the licence types on your vendor’s pricing page: Odoo’s paid plans are priced per user per month, and Business Central has a much cheaper Team Members licence for limited access, such as reading data and approving workflows.

Approvals, pricing or field work happen by email and WhatsApp

A discount is approved in a chat thread, a job is confirmed by phone, a photo arrives by email. For example, a 60-person distributor whose order approvals run by email, or a field-service firm whose technicians fill in paper reports: the ERP records the result, sometimes days later, and the decision trail lives in people’s inboxes.

What to keep packaged

Keep the general ledger, tax, payroll and the filings the law requires in packaged software. Regulation changes without asking you: VAT rules, e-invoicing mandates, payroll thresholds. A vendor with thousands of customers updates them for all of them at once, and your accountant already knows the tool. Rebuilding that is paying to carry a risk that a subscription carries for you.

What the operations layer is, concretely

“Custom module” says little. Here is what the layer holds, piece by piece, in one system your team opens every day. Each piece can come alone or with the others, and together they are what we build as a custom ERP.

• Daily app
• Approvals and documents
• Portals
• Dashboard
• Invoicing triggers

The daily app, in the office and in the field

The screens your team opens every morning: jobs, orders, schedules, checks, on a desktop or a phone. It follows your steps and your words, so new people learn the work and the tool together (business app).

Approvals and documents generated from your data

A request goes to the right person with the right threshold, then to e-signature. Quotes, contracts and reports come out pre-filled in your branding, from the data already in the system (document generation).

Client and supplier portals

A space in your brand where clients follow their files and suppliers send their catalogues, documents and updates themselves, so nobody re-types them (client portal).

One dashboard with live numbers

Margin per job, late orders, stock at risk, on one screen, from the systems that own each figure, so decisions rest on this morning’s numbers (dashboard).

Invoicing triggers and notifications that go back to the accounting software

When an order is approved or delivered, the operations layer asks the accounting software to raise the invoice. Emails and alerts go out when the data says so, so nobody has to remember (invoicing).

The same layer works across countries. For example, a distributor with teams in several countries can run one back office where each country keeps its own approval thresholds, documents and language, while each country’s accounting stays in the software its accountant uses.

Which system owns which data

One rule settles most arguments: every record has one owner. One system creates it and changes it; every other system reads a copy.

Operations appBuilt

• Order
• Stock
• Approval
• Delivery note

Accounting softwareBought

• Invoice
• Ledger entry
• Tax

On approvalStatus back once a day

For example, a distributor that takes orders from shops and delivers them:

• The order lives in the operations app. It is created, priced, approved and delivered there.
• The stock figure lives where stock moves are recorded: the operations app, where the warehouse scans what comes in and goes out.
• The invoice lives in the accounting software. When an order is approved, the operations app sends what to bill; the accounting software gives the invoice its number, its tax and its payment status.
• The payment status flows back to the operations app once a day, so the dashboard shows which orders are paid without anyone opening the accounting software.

Write this list down before anything is built. Most double entry, and most “which number is right?” meetings, come from records with two owners.

What it really costs to own each option

Compare five years of every cost, and model each option at the size you expect to be in year five.

Building is cheaper than it was; running in production isn’t

A working prototype of an operations app now takes days, often with AI writing much of the code. What still costs money is everything that lets a team rely on it: everyone signs in with their own account and sees only what they should; backups exist and a restore has been tested; someone is told when something breaks; changes go out the same way every time, with a way back; syncs with the accounting software retry and report failures. That is the part to price, and the part to ask any builder about. Our prices and the audit are on the services page.

How to stay free to leave a custom build

“You own the code” is easy to promise. Ask how. Three things make it true:

• Your own accounts from day one. The hosting, the database and the code repository are in your company’s name, and the builder is a guest in them.
• Updates you can repeat and undo. A change goes out the same way every time, built by a machine, and the previous version is one step away.
• Decisions written down. Why the data is shaped this way, why that sync runs at night: written next to the code, so the next engineer, or your own AI assistant, can read why.

With those three, you can leave and it keeps working. Maintenance becomes a choice: hand it over with training, keep us for a transition, or keep us on a retainer.

How to decide in two weeks, not six months

1. List the five transactions your team does most often: an order, a job, a quote, a return, a purchase.
2. Ask a packaged vendor to run each one live in a demo, with your data and your exceptions.
3. Score each gap: works as is, works with configuration, needs custom code, or forces a workaround.
4. Prototype the worst one, in days, as a small app connected to your accounting software.
5. Decide: buy, build that one piece, or both.

If you want a second opinion on the list, book the call below. We will tell you which side of the line each transaction falls on.

Once your data is in one place

When the operations data lives in one clean system, with clear owners, your team can also connect Claude or ChatGPT to it, to ask questions in plain words or draft the next step for approval. That is a separate service we run, with its own permissions on what the assistant may read and do.

Frequently asked questions

Talk it through with us

Bring your list of workarounds. By the end you’ll know what to buy, what to build, or whether we can’t help, and if we can’t, we won’t follow up.

Book a free 30-minute call. No commitment. You pick the time.