Technical co-founder or fractional CTO, and how to tell which you need
They do almost the same work. What differs is what you pay with, how long it lasts, and who carries the risk if it goes wrong.
People ask me this question having already decided the answer, and they’re mostly wrong about which one they decided on.
The work is nearly identical. Both roles decide what gets built, choose the architecture, set engineering standards, and sit in the room when a technical decision has commercial consequences. Watch two weeks of either and you’d struggle to tell them apart.
The difference is the deal.
The real difference
A fractional CTO is capability you rent. You pay a fee, you get technical ownership at an agreed cadence, and either side can end it with notice. The risk stays with you. If the company fails, they have been paid.
A technical co-founder is capability you buy a share of the company with. They take equity, rarely a small amount of it, and in exchange they carry your risk alongside you. If the company fails, they worked for nothing.
Everything else follows from that one distinction.
| Fractional CTO | Technical co-founder | |
|---|---|---|
| You pay with | Fees | Equity, 20–50% in most deals I’ve seen |
| Ends when | Either side gives notice | Vesting finishes, or a leaver clause bites |
| Carries your risk | No | Yes |
| Reversible | Yes | Only if you set it up to be |
| Right when | You know roughly what you need | You’re still working out what the company is |
The question that separates them
Not “which is cheaper”. Ask this instead:
Do I need someone to own the technology permanently, or do I need someone to own it now?
Most people who ask me need it now. They have a business, or the clear shape of one, and the technology is either blocking them or about to. What they need is for someone senior to make a series of decisions over the next few months and be accountable for them. That’s a fee, and a fee is dramatically cheaper than equity for that job.
A smaller number need a co-founder. The tell is that they can’t yet describe what the company builds. Not “we know what to build and need someone to build it”, which is a hiring problem. I mean the product question is still open, and answering it needs someone technical sitting inside the business for years, taking the same downside as everyone else.
If you can write a coherent brief, you don’t need to give away half your company to have it executed.
What equity really costs
Founders underestimate this because they price it against today’s valuation, which is low or notional.
Twenty per cent of a company that exits at £5 million is £1 million, for a role that as a retainer might have cost £60,000 a year. If the engagement lasts two years and the company takes six to exit, you paid roughly eight times the market rate.
Vesting is what stops that arithmetic being automatic, and it’s the part founders skip. Standard UK terms put a founder’s equity on a vesting schedule with good and bad leaver provisions, so someone who leaves in year two keeps a slice and not the whole grant, and some structures let the company buy it back. The protection is normal, it’s cheap to draft, and asking for it isn’t an insult. Get it agreed before anyone signs, because nobody has ever cheerfully agreed to it afterwards. Where founders get hurt is the grant with no schedule attached, and that’s a drafting failure. Nothing about equity requires it.
None of that’s an argument against co-founders. Sometimes it’s exactly the right trade, because the alternative is no company at all. It’s an argument against reaching for equity because it feels free on the day you sign. It’s the most expensive currency you have and the only one you can’t earn more of.
There’s a version of this that goes wrong in the other direction too. I’ve had investors on a cap table and taken them off again, and the second part is far harder and more expensive than the first. Whoever you give equity to, you marry their circumstances as well as their contribution: their divorce, their next job, their view in three years about whether they still got a fair deal.
The middle, where most engagements land
The framing above is a clean binary. Real engagements seldom are.
Every engagement I’ve taken started as a fixed fee. Where equity came into it, the client raised it, and they raised it afterwards, once a fee had already shown the work was worth having. That sequence matters. A fee proves the value before anyone commits to something irreversible, and it means both sides negotiate from evidence instead of optimism.
So if you’re considering equity, consider it second. Start with a scoped, paid piece of work, small enough that being wrong about each other is cheap. If it goes well, the equity conversation is better informed and much less fraught. If it does not, you lost a few weeks and some money instead of a fifth of your company.
Telling which one you’re
Some honest self-diagnosis.
You need a fractional CTO if: you can describe what the business does; the technical decisions are the blocker, not the identity of the company; you have budget or can get it; and you’d like to be able to change your mind in six months.
You need a technical co-founder if: the product question is open; you can’t pay a market rate and won’t be able to for some time; the technology is the company instead of supporting it; and you’re prepared to run the search for as long as it takes, because good ones are scarce and the wrong one is worse than none.
You need neither if: the spec is written and you want it built. That’s a development team, and paying leadership rates for it wastes your money and their time.
The part nobody says
The people best suited to being a technical co-founder are, by definition, the people with the most options. They can take a senior salary, run their own consultancy, or found something themselves. Persuading one to take your risk instead needs either an unusually compelling business or an unusually strong existing relationship. Searching for one on the open market has a poor hit rate for exactly that reason.
Which is why so many founders end up looking for a fractional CTO having started out looking for a co-founder. I don’t count that as a compromise. They reached the right answer slowly.