How we built a complete client portal for tax advisory firms

A growing Dutch tax advisory firm faced a problem many advisors recognize: the entire process — from initial client contact to filed tax return — ran through email, paper forms, and scattered Excel files. Documents got lost, clients called for status updates, and invoices were sent late.

They needed one platform that digitizes the complete workflow, is secure enough for sensitive national ID numbers (BSN), and simple enough for their clients to use independently.

Our Solution: BAS Portal

We built a full-stack platform with three interfaces: an Advisor Dashboard (Next.js web application) for advisors and administrators, a Client Portal where clients upload documents and pay invoices, and a Mobile App (React Native/Expo) for access on the go.

Lead-to-Client Pipeline

When a potential client fills out the contact form on the website, it immediately enters the system as a lead. The advisor can assign the lead, reach out via email templates, and convert it to a full client with one click — including automatic client profile creation.

Digital Case Management

Each tax year gets its own case (dossier) with a clear 5-step workflow that guides the client through the portal:

• Portal Access — create account via invitation
• Upload ID Document — securely through the portal
• Tax Authority Authorization — upload the VIA authorization form
• Prepare Declaration — the advisor does their work
• Review & Approve — client gives digital approval

BSN Encryption and GDPR Compliance

Because we handle Dutch national ID numbers (BSN), security is not an afterthought. BSN is stored encrypted (AES encryption) with HMAC-SHA256 hash for lookups. BSN is masked in API responses. Full GDPR compliance: data export (Art. 20) and anonymization (Art. 17). Two-factor authentication for staff.

Invoicing with iDEAL Payments

• Automatic invoice numbering (INV-2025-0001)
• Dutch VAT rates (0%, 9%, 21%)
• Professional PDF invoices
• Mollie iDEAL integration — clients pay directly online
• Automatic payment reminders for overdue invoices
• Clients view invoices via a secure link (no login required)

Role-Based Access Control

The system features three roles with strict permissions: Administrator (full access), Advisor (only sees their own clients and cases), and Client (only sees their own data). This extends to the object level: an advisor cannot access files from a case assigned to a colleague.

The Result

• 80% fewer emails about document status
• Faster turnaround through the digital approval process
• Immediate payments via iDEAL instead of weeks of reminders
• Fewer errors through structured document uploads
• Happier clients who can check their status 24/7

We build similar portals for other advisory firms and financial services providers. The platform is modular and can be customized to fit your workflow. Get in touch for a demo or no-obligation conversation.