Custom Client Portal for an Accounting Firm in London: 2026 Cost vs SaaS

The verdict in three sentences

For an accounting firm in London with 400 to 800 clients, a custom client portal costs EUR 18,000 to 45,000 (about USD 19,500 to 49,000) excluding tax and becomes cheaper than SaaS billed at EUR 3 to 8 per client per month from the third year. The real gain comes from integration with the practice software (Cegid or ACD in France, IRIS or CCH in the UK): without it, the portal is just one more upload box. Aim for a realistic gain of one hour per client per month on bookkeeping clients, not across the whole portfolio.

Off-the-shelf SaaS or custom portal

Document collection and client relationship SaaS tools are fast to roll out, but their cost grows with the number of clients and their connectors stay generic. Custom software needs an upfront investment and a solid vendor, but it fits the firm's procedures.

5-year cost comparison for 600 clients

2026 estimate, excluding tax. Below 200 clients, SaaS usually remains the rational choice: at EUR 3 per client, it costs under EUR 38,000 over 5 years.

The scope that actually saves time

A useful portal fits in five functions: drag-and-drop and mobile photo upload, automatic matching with the accountant's document requests, scheduled reminders, a vault for produced documents (accounts, tax returns, payslips) and engagement letter signature. The rest (internal messaging, cash dashboard) can wait for version two. The practice software connection must at least pull the client list and push documents into the client's file storage.

On compliance, in-country hosting, encryption and access logging meet professional body and GDPR expectations. Plan two-factor authentication for clients, otherwise shared accounts multiply.

Mini case study

Sophie, managing partner of a London accounting firm with 600 clients, 200 of them on monthly bookkeeping, exchanges documents by email and chases by phone. She chooses a custom portal at EUR 30,000 with a practice software connector. On the 200 bookkeeping clients, her team saves 1 hour per client per month on collection and filing, or 2,400 hours a year. Conservatively assuming only half of that time is redeployed to billable work at a EUR 45 hourly cost, the gain reaches 1,200 h × EUR 45, or EUR 54,000 (USD 58,500) a year. The portal pays for itself in under 7 months, maintenance included.

FAQ

How long does it take to build an accounting client portal? Plan 3 to 5 months for a complete first version, including 4 to 6 weeks for practice software integration. A gradual rollout on 50 pilot clients is recommended.

Will our clients actually use the portal? Adoption usually exceeds 70 % after 6 months if mobile photo upload is simple and the firm stops accepting documents by email. Without that rule, it plateaus around 40 %.

Is an e-signature legally valid for an engagement letter? Yes, a simple or advanced electronic signature compliant with eIDAS (or UK eIDAS) is enough. Unit cost ranges from EUR 1 to 2 depending on provider and volume.

Who owns the code of a custom portal? The contract must assign the rights to the firm and hand over the source code. That is the condition for changing vendors without starting over, with reversibility costed at about 5 % of the initial budget.

Can we start with SaaS and migrate later? Yes, provided the vendor allows a full export of documents and metadata. Check before signing: a poorly prepared migration costs EUR 3,000 to 8,000.

Let's scope your project. Tell us your number of clients, your practice software and current workflows: we will price a custom client portal, indicative budget EUR 18,000 to 45,000, delivered in 3 to 5 months with a 50-client pilot. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.