Most agency selection processes are backwards

Most founders who are choosing a SaaS development agency spend too much time evaluating websites and not enough time evaluating engineers. They look at portfolios, read testimonials, and make decisions based on design quality and sales competence — none of which directly predict whether the technical work will be any good.

This guide is about evaluating agencies the right way. It will help you identify good agencies and spot the red flags that suggest you should keep looking.

Start with the problem, not the solution

Before you approach any agency, get clear on exactly what you are trying to build and what a successful engagement looks like. "Build my SaaS" is not a brief. "Build a multi-tenant property management platform that handles rent collection, maintenance tracking, and tenant communication, with an MVP ready in 12 weeks" is a brief.

Agencies that are happy to proceed with a vague brief are typically either very junior or planning to expand scope later. Good agencies will ask a lot of questions upfront. Treat the quality of those questions as your first data point.

Technical evaluation matters more than portfolio

Portfolios tell you what an agency has worked on. They tell you very little about code quality, engineering decisions, or what the engagement was actually like. A glossy case study was written by the agency's marketing team, not by the client's engineers.

Instead, ask for a technical conversation with the engineers who would actually work on your project. Come with questions:

How would you approach multi-tenancy for a product like mine?
What is your preferred approach to database migrations in production?
How do you handle a situation where client requirements conflict with good architecture?
What does your CI/CD setup typically look like?

You do not need to be a technical expert to evaluate the answers. What you are looking for is whether they engage thoughtfully with the questions, or whether they give vague reassurances. Vague reassurances are a red flag.

Ask for references from technical stakeholders

Agency references are usually pre-vetted — they will connect you with happy clients. That is fine, but make sure you speak to someone technical at the reference company, not just the founder or commercial lead. Ask them:

What was the code quality like?
Did the agency push back when they disagreed with your technical decisions?
Were there any surprises in scope or cost?
Would you hire them again?

The last question is the most revealing. "Yes, definitely" and "probably, for certain types of work" are very different answers.

Understand who will actually work on your project

A common pattern in agencies: senior engineers are involved in the sales process, then junior developers are assigned to the project. This is not necessarily a problem — all agencies have a mix of experience levels — but you should understand the composition of your team upfront.

Ask specifically: who will be working on my project day to day? What is their experience? Will a senior engineer be reviewing code? How much involvement will senior staff have?

If the answers are vague or uncomfortable, that tells you something.

Red flags to watch for

Suspiciously low quotes

A SaaS MVP from a quality London agency costs £25,000–£50,000. If you get a quote for £8,000, something is wrong. Either the scope has been misunderstood, the work will be done by very junior developers, or the quote will expand significantly once work begins. Get detailed breakdowns, not ballpark figures.

Guaranteed fixed-price quotes based on brief conversations

Fixed-price projects require very precise specifications. An agency that quotes a firm price after a 30-minute call has either done so many times they can estimate confidently, or they are quoting high to cover unknowns, or they are setting you up for change request conversations later. Ask how they handle scope changes.

No interest in your business, only in the technical requirements

Good agencies want to understand why you are building this, who your users are, and what success looks like. An agency that jumps straight to technology choices without understanding the business context is likely to build something technically sound that does not actually serve the business.

Reluctance to discuss past failures

Every agency has had projects go sideways. How they handled those situations tells you a great deal about their values and processes. An agency that claims everything has always gone smoothly is either very lucky or not being honest with you.

Evaluating contracts and commercial terms

Before signing anything, make sure you understand:

IP ownership: You should own all code produced for your project on final payment.
Payment terms: Milestone-based payments are reasonable. Asking for 100% upfront is not.
Scope change process: How are changes to scope handled? Who decides, and who approves the cost?
Termination clauses: What happens if you need to end the engagement early? What code and assets are you entitled to?
Confidentiality: NDA before sharing sensitive business information is reasonable to request.

The right agency relationship is a partnership

The best agency engagements feel like a partnership — where the agency challenges your thinking, brings ideas to the table, and cares about the outcome, not just the deliverables. If the relationship feels transactional from the first conversation, it will probably remain transactional throughout the project.

Look for an agency that asks questions you had not thought to answer, points out risks in your plan without being asked, and seems genuinely interested in whether the product succeeds. Those are the signals that matter most.

How to Choose a SaaS Development Agency in London