The verdict in three sentences
For a software vendor moving from one install per customer to SaaS, a shared database with row-level security (RLS) is the best default up to a few hundred customers. Strong isolation (one database per tenant) adds 20 to 40% to development cost and is only justified for regulated customers or contracts that require it. Plan a rework of EUR 60,000 to 150,000 (about USD 65,000 to 160,000) over 5 to 8 months and infrastructure of EUR 300 to 2,000 a month for 100 tenants.
The three isolation models compared
The isolation model drives everything else: migrations, backups, hosting cost and the sales pitch to customer IT departments.
| Criterion | Shared database + row-level security | Schema per tenant | Database per tenant |
|---|---|---|---|
| Development premium | Baseline | +10 to 20% | +20 to 40% |
| Infrastructure for 100 tenants | EUR 300 to 600 a month | EUR 500 to 1,000 a month | EUR 1,200 to 2,000 a month |
| Schema migrations | Single run | 100 orchestrated runs | 100 orchestrated runs |
| Cross-tenant leak risk | Controlled by tested RLS policies | Low | Very low |
| Restoring a single tenant | Complex (filtered export) | Simple | Very simple |
| Enterprise sales argument | Average | Good | Excellent |
| Practical ceiling | Several thousand tenants | 500 to 1,000 schemas | A few hundred |
The hybrid model is becoming standard in 2026: shared database for SMBs, dedicated database sold as an "Enterprise" option billed EUR 300 to 800 more per month.
The rework budget line by line
The rework touches far more than the database. Authentication, billing and tenant administration are often underestimated.
| Line item | Scope | Indicative 2026 budget |
|---|---|---|
| Data model and isolation | Tenant column, RLS policies, leak tests | EUR 12,000 to 30,000 |
| Authentication and roles | Organizations, invitations, per-tenant SSO | EUR 8,000 to 18,000 |
| Subscription billing | Plans, quotas, Stripe, invoices | EUR 8,000 to 20,000 |
| Admin console | Tenant creation, support, audited impersonation | EUR 6,000 to 15,000 |
| Migrating existing customers | Scripts, data transfer, cutover in waves | EUR 10,000 to 30,000 |
| Observability and CI/CD | Per-tenant logs, alerts, continuous deployment | EUR 6,000 to 15,000 |
| Security and load testing | Penetration test, multi-tenant tests | EUR 10,000 to 22,000 |
A tight rework on a shared database lands around EUR 60,000. With dedicated databases, per-tenant SSO and migration of 100 installs, it reaches EUR 150,000.
Timeline and pitfalls to avoid
A realistic timeline: 1 month of architecture, 3 to 5 months of development, 1 to 2 months of progressive customer migration. Recurring pitfalls: forgetting the tenant filter in scheduled jobs, caching data without a tenant key, and keeping customer-specific code instead of turning it into settings.
Simple rule: any customization that concerns more than 3 customers becomes a configuration option, the others are declined or billed.
Mini case study
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Thomas, CTO of an HR software vendor, maintains 100 installs on dedicated servers at EUR 90 a month each, or EUR 9,000 a month, plus manual updates adding up to 40 person-days a month. He picks a shared database with RLS and a dedicated-database option for 8 customers. Rework: EUR 95,000 over 6 months. New infrastructure: EUR 1,100 a month. Hosting savings: EUR 7,900 a month, or EUR 94,800 a year. The project pays back in 12 months, before counting the 30 person-days a month freed up.
FAQ
Which isolation model should a new SaaS start with?
A shared database with row-level security covers 80% of cases up to several thousand tenants. Keep a dedicated database as a paid option for regulated accounts.
How much does turning installed software into SaaS cost?
Budget EUR 60,000 to 150,000 depending on the isolation model, the number of customers to migrate and billing. Timeline is 5 to 8 months.
Is row-level security enough for GDPR?
Yes, if policies are tested automatically on every deployment and access is logged. A multi-tenant penetration test at EUR 6,000 to 12,000 gives customers the proof.
What does infrastructure cost for 100 tenants?
EUR 300 to 600 a month on a shared database, up to EUR 2,000 a month with one database per tenant. Backups and monitoring account for 15 to 20% of that amount.
Can customers be migrated gradually?
Yes, in waves of 10 to 20 customers over 1 to 2 months. This limits risk and lets you adjust migration scripts between waves.
Let's scope your project. Tell us your customer count and current architecture, and we will price a multi-tenant rework between EUR 60,000 and 150,000 with a 5 to 8 month plan. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.