Business softwareCustom softwareIntegrationSMEs
Building a custom client portal
6 min read
A custom client portal is a secure online space where your customers find their documents, follow their cases, accept or sign quotes and pay, connected to the systems you already run. Its cost depends less on the number of screens than on three factors: integrations (accounting, ERP, payments), how granular the access rights are, and how much of your own business logic it has to reproduce. Before commissioning one, check that an off-the-shelf portal would not do the job.
This guide covers the features worth having, what moves the budget, the security to insist on, and a way to launch without over-building.
What a client portal actually fixes
A portal is not about looking modern. It answers specific frustrations: customers calling to ask where their case stands, quotes sent as attachments and lost, invoices requested again and again, signed documents circulating by email with no single reference version.
The payoff is time saved on both sides: customers find answers on their own, at any hour, and your team stops answering the same questions. It is also traceability: every accepted quote, uploaded document and payment is timestamped in one place.
Features, from core to optional
| Feature | What it gives you | Complexity |
|---|---|---|
| Document space | Invoices, contracts, reports in one place | Low |
| Case or order tracking | Current status without a call | Medium, depends on your stages |
| Online quotes | Review, acceptance, history | Medium |
| E-signature | Remote, traceable approval | Medium, through a provider |
| Online payment | Faster collection | Medium, through a provider |
| Messaging | Conversations tied to the case | Medium |
| Accounting sync | Invoices and payments without re-keying | High, depends on the tool |
A useful first release often fits in three features: the document space, case tracking and quotes. The rest arrives in stages, once real usage shows what is missing.
Signatures and payments: integrate, do not reinvent
Two features are never built from scratch; they are integrated.
Electronic signatures. In the European Union, the eIDAS regulation (EU) No 910/2014 states in Article 25 that a signature cannot be denied legal effect solely because it is electronic, and gives a qualified electronic signature the same legal effect as a handwritten one. In the United States, the ESIGN Act likewise says a signature or contract "may not be denied legal effect, validity, or enforceability solely because it is in electronic form". For a routine quote, a standard e-signature from a reputable provider is usually proportionate; the highest levels of assurance are for high-stakes documents. The portal calls the provider, which keeps the audit trail.
Payments. A portal collects money through a payment provider, which carries the card data compliance burden. Fees are per transaction: as one example, Stripe lists 1.5% + €0.25 for standard European Economic Area cards on its European pricing, with different rates in other regions. These fees come on top of the portal itself, and should be weighed against the time your team spends chasing payments today.
Accounting integration and e-invoicing
This is usually the most underestimated line. A portal that shows invoices has to read them from somewhere; a portal that takes payments has to send them back to your accounting system, without duplicates or re-keying. Each connection must handle errors, the other system being down, and edge cases such as credit notes, deposits and partial payments.
E-invoicing rules add to the picture in a growing number of countries. France is a clear example: since 1 September 2026, every VAT-registered business established there must be able to receive electronic invoices through an approved platform, and the obligation to issue them extends to small and medium-sized businesses on 1 September 2027 (schedule on economie.gouv.fr, in French). Where such rules apply, the portal does not replace the official channel: it displays and circulates documents, while the legal e-invoice leaves through your invoicing software or platform. The specification should say clearly which system issues what.
Security: the part you cannot trade away
A client portal exposes your data to the outside world by design. The core rule is simple to state: each customer only sees what concerns them, and the server checks it on every request.
This is not theoretical. In OWASP's reference list of web application risks, broken access control remains in first place in the 2025 edition. The textbook case: change a number in a document's URL and land on another customer's invoice.
The minimum to insist on:
- strong authentication, with a second factor available and a password reset flow that reveals nothing;
- tested permissions, including deliberate attempts to reach another account's data;
- encrypted traffic, tested backups and an access log;
- compliant handling of personal data, with a data processing agreement if your customers are in Europe.
Our guides to business application security and the GDPR checklist for a business application go into more detail.
Custom or off the shelf
| Your situation | Lean towards |
|---|---|
| Standard journey: documents, invoices, messages | Off the shelf |
| Stages specific to your trade, approval rules | Custom |
| Close connection to a business system or ERP | Custom |
| Few customers, need still to be proven | Off the shelf, then reassess |
An off-the-shelf portal costs a subscription, often per user, and fits you to its journey. A custom portal follows your journey, but needs a build and a maintenance budget. Our article on what a custom business application costs explains how such a project is priced, and our guide to subscription versus upfront build compares the two ways of paying over three years.
A four-step method
- Start from the questions your customers ask. The ten most common reasons they call or email you outline the first release.
- Map your data. Where do quotes, invoices and case statuses live today? Each source is an integration to price.
- Make the journey visible. A prototype shows what the customer sees and what your team administers, and becomes the basis for the quote.
- Ship in stages. Documents and tracking first, quotes and signatures next, payments and accounting sync once usage has settled.
A "client portal and invoicing" application is one of the typical projects described on our home page: customer area, online quotes, e-signature, automated invoicing and reminders, connected to the existing accounting tool.
Where to start
List the questions your customers ask you every week and the tools where the answers live: that is the starting point for your portal. We then present a free visual prototype of the customer area and its admin side in a video call within 5 working days, wherever you are. The three ways to pay for the build are on our offers page.