Technical co-founder vs CTO vs fractional CTO: what's the difference?
Short answer
A technical co-founder, a CTO and a fractional CTO differ in what they own. A technical co-founder owns part of the company and builds it from the start, usually for founder shares and a low salary. CTO is a job title: the executive accountable for technology, held by a co-founder or a hire. A fractional CTO is a part-time senior leader paid by the day or month, who owns agreed outcomes rather than the company.
By Dr. Dulan Dias, co-founder and CTO of Tellme AI
Key facts
- Co-founder is a description rather than a legal position: under UK company law, a technical co-founder's rights come from being a shareholder, a director, an employee or some mix of the three.
- A technical co-founder who sits on the board is a company director, and directors of UK companies owe seven general duties set out in sections 171 to 177 of the Companies Act 2006, including the duty to promote the success of the company.
- Under UK rules, anyone who holds more than 25% of a company's shares or voting rights is a person with significant control, which includes many technical co-founders at incorporation.
- Since 6 April 2021, when a medium or large private-sector client engages a contractor who works through their own limited company, the client is in most cases responsible for deciding whether the off-payroll working rules (IR35) apply; for a small client, the contractor's company decides.
- HMRC's Check Employment Status for Tax (CEST) tool gives HMRC's view of whether a worker is employed or self-employed for tax, and can be used to check whether IR35 applies to an engagement such as a fractional CTO's.
- Dr. Dulan Dias joined Tellme AI as founding engineer in June 2025, became its Chief Technology Officer in December 2025 and was made a co-founder in June 2026: three titles, arriving separately.
Definitions
- Technical co-founder
- A founder who takes responsibility for the technology from the start, usually holding founder shares that vest over time and building the first version of the product personally.
- Chief Technology Officer (CTO)
- The executive accountable for a company's technology: architecture, technical risk, security and the direction of the engineering team. It is a job title, so a co-founder, a promoted engineer or an external hire can hold it.
- Fractional CTO
- An experienced technology leader who works part time for one or more companies, usually on a monthly retainer or a day rate, and typically without founder equity or a board seat.
- VP of Engineering
- The leader accountable for engineering execution: hiring, management, delivery process and team health. Growing companies often separate it from the CTO role, which then concentrates on technical strategy.
- Off-payroll working rules (IR35)
- UK tax rules for contractors who work through their own company but would be employees if engaged directly. They decide how income tax and National Insurance are paid on the engagement.
Technical co-founder vs CTO vs fractional CTO at a glance
The three differ most in what each one owns. A technical co-founder owns part of the company, a hired CTO owns a function, and a fractional CTO owns a set of agreed outcomes for as long as the engagement lasts. The rest of the table follows from that.
| Factor | Technical co-founder | Hired CTO | Fractional CTO |
|---|---|---|---|
| What it is | An owner who builds the company | An executive role with a salary | A part-time senior leader or interim adviser |
| Equity | Founder shares, normally vesting over several years | Share options or a small share grant | Usually none, sometimes a small option grant |
| Commitment | Full time and open-ended | Full time, under an employment contract | A set number of days a month, ended on notice |
| Typical stage | Idea to seed | Seed onwards, once there is a product and a team to lead | Any stage with a scoped need, often pre-seed or between leaders |
| Accountable for | The product and the company's survival | Technology strategy, the team and technical risk | The deliverables written into the contract |
| Cost model | Low or deferred salary, paid mostly in equity | Market salary plus options | Day rate or monthly retainer |
| UK legal position | Shareholder, usually a director, sometimes also an employee | Employee, sometimes also a director | Contractor, often through their own company, so IR35 needs checking |
| How it ends | Leaver terms in the shareholders' agreement | Notice under the employment contract | Notice under the services contract |
Is a technical co-founder the same as a CTO?
No. Co-founder describes how someone joined and what they own; CTO describes the job they do. A technical co-founder often takes the CTO title on day one because someone has to own the technology, but the title should follow the work rather than the share register.
The two can arrive separately and in either order. Dr. Dulan Dias joined Tellme AI as its founding engineer in June 2025, became Chief Technology Officer in December 2025 and was made a co-founder in June 2026. The reverse also happens: a co-founder hands the CTO title to a hire and stays on as an owner in a different role.
For what a technical co-founder actually does at each stage, from the idea to the first hires, see what does a technical co-founder do.
When does each one fit?
Choose by what the company needs over the next two years, not by which title sounds most senior.
- A technical co-founder fits when the technology is the product or the advantage: an AI-native product, deep tech, or anything that needs years of daily technical decisions. It also fits when the company cannot yet pay market salaries. Whether you need one at all is covered in do I need a technical co-founder.
- A hired CTO fits once there is funding, a working product and a team to lead, or when the company needs experience its founders lack, such as running a larger engineering organisation or selling to regulated customers.
- A fractional CTO fits a scoped need: choosing a stack, hiring the first engineers, overseeing an agency, preparing for technical due diligence, or covering the gap between technical leaders. It does not fit a product that needs someone thinking about it every day.
What can a fractional CTO do, and where does it fall short?
A good fractional CTO brings judgement an early team cannot afford full time: architecture reviews, a hiring plan, agency and vendor oversight, security basics and an honest view of what the product needs next. The limits come with the model. Attention is part time, knowledge leaves when the engagement ends, and nobody working two days a month can carry a product the way an owner does.
Make the engagement work by writing down deliverables, keeping every repository, cloud account and domain in the company's name, and asking for decisions to be documented as they are made.
In the UK, when a fractional CTO works through their own limited company, the off-payroll working rules (IR35) have to be considered: they apply if the person would have been an employee had the company engaged them directly. HMRC's Check Employment Status for Tax tool gives HMRC's view on a particular engagement. This is tax territory, so take advice.
How does a technical co-founder's role evolve into CTO or VP of Engineering?
A technical co-founder's job changes several times, and each change asks them to stop doing something they are good at. Watch for the signals in each step rather than a headcount, because teams reach them at different sizes.
- Builder. The co-founder writes most of the code and makes every technical decision. The risk is building alone for too long.
- First hires. Time moves to hiring, code review and keeping the architecture coherent. The signal to move on is that they can no longer review everything that ships.
- Team lead. Delivery now depends on how well others work, so priorities, one-to-ones and unblocking take most of the week. How to scale an engineering team covers what changes as teams multiply.
- The fork. Eventually the job splits. The CTO path keeps technical strategy, architecture, security and the technical conversation with customers and investors; the VP of Engineering path keeps people, process and delivery. Many technical co-founders pick one and hire for the other, and neither is a demotion. Some take a third route and run the company: Dr. Dulan Dias, a computer scientist by training, founded EchonLabs in 2017 and led it as founder and CEO until 2025, through growth to a team of 60+ and its acquisition by AINSUNHZ Global Technology Pte. Ltd. in 2026.
Equity, vesting and the legal position in the UK
Titles carry no legal weight on their own; shares, directorships and contracts do. A co-founder who holds more than 25% of the shares or votes is a person with significant control. A co-founder on the board is a director, with the seven general duties in sections 171 to 177 of the Companies Act 2006, including promoting the success of the company and avoiding conflicts of interest.
A hired CTO is normally an employee, often with share options. A fractional CTO is normally a contractor under a services agreement. Whichever you choose, make sure the company owns the code everyone writes; technical due diligence explains why investors check written IP assignments.
How much equity a technical co-founder should get, and how vesting, cliffs and leaver terms work, is covered in technical co-founder equity. None of this is legal or tax advice.
Common mistakes when choosing between them
- Giving co-founder equity to someone who will only work part time. Equity should buy commitment; part-time help is better paid in cash or a small option grant.
- Calling a contractor the CTO and expecting an owner's commitment. A title does not change incentives.
- Hiring a CTO from a large company too early. Running an organisation of hundreds is a different job from building the first version of a product.
- Keeping a co-founder in the CTO seat after the job has outgrown their interest. A move to architecture, product or engineering delivery is often right for both sides.
- Leaving the company's code, cloud accounts or domain in a contractor's personal name.
Frequently asked questions
Can a fractional CTO become a co-founder?
A fractional CTO can become a co-founder if both sides want a permanent commitment, and a short paid engagement is a sensible way to test that. Agree in advance what the change would mean: full-time work, founder shares on a vesting schedule and an end to other clients. Do not convert past retainer days into founder equity without a written agreement, because it confuses ownership when the next investor looks.
Should a technical co-founder always take the CTO title?
A technical co-founder does not have to take the CTO title. Early on it is usually the honest description, because someone has to own the technology. Later, the co-founder may be better suited to leading architecture, product or engineering delivery, and a hire may make a better CTO. Investors care more about whether the technical leadership fits the stage than about who holds which title.
What is the difference between a CTO and a VP of Engineering?
A CTO is accountable for technology strategy: architecture, technical risk, security and the technical conversation with customers and investors. A VP of Engineering is accountable for execution: hiring, managing engineers, delivery process and team health. In a small startup one person does both. As the team grows the two jobs pull apart, and many companies split them so each gets full attention.
Do investors accept a fractional CTO instead of a technical co-founder?
Investors in a software or AI startup usually want to know who will own the technology long term, and a fractional CTO rarely answers that on its own. A fractional CTO can strengthen a pre-seed company by setting sound foundations and helping it hire, but expect investors to ask how and when a permanent technical leader will join, and what that person will own.
Is a fractional CTO caught by IR35?
A fractional CTO who works through their own limited company may be. The off-payroll working rules apply where the person would have been an employee if engaged directly. Since April 2021, a medium or large private-sector client is generally responsible for deciding; a small client leaves the decision to the contractor's company. Control, substitution and how integrated the person is all matter, so check each engagement and take advice.
When should a startup hire a full-time CTO?
A startup should hire a full-time CTO when technology decisions need daily ownership that nobody currently provides: typically after funding, with a product in use, a team to lead and a roadmap that depends on technical judgement. If a technical co-founder already does that job well, the better hire may be a VP of Engineering to run delivery as the team grows.
Can a startup have a CTO who is not a co-founder?
A startup can have a CTO who is not a co-founder, and many do, either by hiring one after a funding round or by promoting an early engineer. The difference is ownership. A hired CTO is usually an employee with share options rather than founder shares, so their incentives and leaver terms differ from a co-founder's. Both sides should be clear about that from the start.
Sources
About the author
Dr. Dulan Dias is co-founder and Chief Technology Officer of Tellme AI in Belfast; founder of EchonLabs, grown from 2017 to more than 60 people before its acquisition in 2026; holder of a PhD in AI and data science from Queen’s University Belfast; and the author of From Model to Product.