Odoo and Custom Systems for UK Contractors

Why UK contractors outgrow their accounts package

The story we hear most goes like this. A contractor grows from a handful of jobs to fifteen live sites. Sage or Xero still does the books, but the commercial side has moved into spreadsheets: one for each valuation, one for retentions owed and held, one tracking which subcontractors were verified with HMRC and at what rate. The monthly CIS return is pieced together from bills. Nobody can say what a job will make until the final account.

None of this is anyone being careless. Generic accounting software is built around invoices and ledgers, while a contractor earns money through contracts, valuations and certified sums. The pages below take each part of that gap in turn.

United Kingdom guides

The UK rules that decide what your system has to do

Four sets of rules sit behind most of the spreadsheets we replace. Each one needs data your accounts package may not hold.

• Construction Industry Scheme (CIS). Contractors deduct 20% from registered subcontractors, 30% from unregistered ones and nothing from those with gross payment status, after verifying them with HMRC (gov.uk). From 6 April 2026 contractors must again file a nil return or tell HMRC about inactive months, and a late return draws a £100 fixed penalty (CIS 340).
• VAT domestic reverse charge. Since 1 March 2021, most CIS-reported services between VAT-registered businesses are invoiced without VAT, and the customer accounts for it, unless the customer is an end user or intermediary supplier (HMRC technical guide).
• Construction Act payment rules. The Housing Grants, Construction and Regeneration Act 1996 gives a right to stage payments on work lasting 45 days or more, requires payment notices and pay less notices, and makes pay-when-paid clauses ineffective outside insolvency (s109, s110A, s113).
• Retentions. Not a statute but a contract term: the JCT default is 3%, often raised to 5%, with half released at practical completion (BESA).

Compliance dates on the horizon

Two digital tax programmes also affect the software decision, though less directly than CIS.

E-invoicing matters to contractors because applications for payment, valuations and reverse charge invoices will have to leave your system as structured data rather than a PDF. A system chosen in 2026 should be able to do that by 2029.

What Devvista does for UK firms

We work in two ways, often on the same project.

Odoo set up for construction

Odoo's UK localisation ships a CIS module that sends the monthly return to HMRC and produces the CIS deduction report, alongside MTD VAT and BACS payment files (Odoo 19 UK documentation). We add the construction layer: jobs as projects with their own cost codes, subcontract orders, valuations, retentions and reverse charge taxes. Our Odoo implementation service describes the method in detail.

Custom software where Odoo is the wrong tool

Some firms need a site app for daily diaries and labour booking, a subcontractor portal for applications, or a link between an estimating tool and the ledger. We build those as custom software and connect them to Odoo or to the accounts package you keep.

Five steps to move a contractor off spreadsheets

1. List every spreadsheet and who owns it. Valuations, retention log, CIS tracker, plant hire, cash flow. This list becomes the scope.
2. Agree a cost code structure that estimating, site and accounts will all use. Without it, job costing never lines up.
3. Set up CIS and reverse charge first and test them against a past month's return and VAT period before anything else goes live.
4. Load live jobs only, with budget, certified to date, retention held and subcontract commitments. Finished jobs stay in the old system for reference.
5. Run one month in parallel, then switch at the start of a CIS tax month (the 6th) so returns are never split across systems.

The detail for each step is on the construction ERP page.

How you know the new system is working

• CIS return time: from a day of reconciling bills to a review of a report Odoo produces.
• Days to month-end: measure before and three months after go-live.
• Margin visibility: can a director see forecast margin per job without asking the QS?
• Retention aged list: every retention owed to you, with its release date, in one report.
• Spreadsheets retired: count them against the list from step 1.

Common mistakes when UK contractors change systems

• Buying a US construction platform and discovering CIS and reverse charge are missing.
• Treating retentions as a note on the invoice rather than a balance with a due date.
• Migrating closed jobs and old suppliers, which doubles the data clean-up.
• Leaving subcontractor verification outside the system, so the deduction rate on the bill is a guess.
• Choosing software in 2026 with no plan for structured e-invoices in 2029.

Is Devvista the right fit for your firm?

Devvista is an official Odoo partner that implements Odoo for UK main contractors and specialist subcontractors who want CIS, job costing, retentions and applications for payment in one system, and builds custom tools around it where Odoo stops. We work remotely during UK business hours, by video call and screen share. We have no UK office. Our Odoo projects are under NDA, so we walk through anonymised examples on a call rather than naming clients.

Good fit: 10 to 200 staff, several live jobs, a finance lead or commercial manager who can give the project time. Not a fit: a sole trader who only needs CIS on Xero, or a large firm already committed to a construction suite such as COINS.

Where to go next

• Construction ERP for UK contractors: the full picture, one system for CIS, job costing and subcontractors.
• Construction accounting software in the UK: Sage, Xero with add-ons, construction suites and Odoo compared.
• Construction job costing and cost control: cost codes, committed costs and CVR.
• Construction budgeting software: budgets, cost-to-complete and cash flow forecasts.
• ERP for manufacturing: Odoo for small makers, covering stock, bills of materials, work orders and the move off Xero or Sage.
• ERP cost for a small manufacturer: a worked example in pounds, a timeline and Made Smarter explained.
• How Odoo pricing works and when to customise Odoo.