Redistribution of system similar to the one commissioned (privately) by a client

Question

The short: my company have been paid to develop a software system for a client. There are (sort of) similar systems on the market already, there is currently a lot of demand for it, I want to redistribute something similar if I am within my rights to do so.

I am not employed by the client, nor has anything formal been agreed (in terms of ownership) - but I have developed a system for them according to their design and requirements. I have not sold them this product as an off-the-shelf option, it is entirely bespoke.

There are some features of this system that are arguably too bespoke, and technical approaches I have taken due to their own constraints, so if I were to aim to package it up for redistribution/sale to other customers, it would most definitely be different enough to the original, in many ways.

My questions (with regard to my rights to sell it elsewhere as a product):

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Does 'different enough' matter? I am assuming it does, but I do not know the law as to whether the intellectual property is theirs until the product varies from what they paid for? Am I concerned with the right thing here? How different must my product be? Is this quantified anywhere in law?

Should I be concerned at all since nothing was in writing? Or are legal implications already made as a result of them paying the invoices I send them?

What else am I not considering here? I have no clue about the legals with regards to this, but I am sure it must be well covered. I've signed contracts in the past (with other clients, in totally different industries) stating that I may not work for a competitor etc. within 6 months after project completion, but nothing has been formalised here.

EDIT: Apologies, I should clarify - I am based in the UK, so UK law is what I'm concerned with.

Answer 1

I take it that you were hired for a one-time job to create this software for the client, what would be called a contracting job in the US, and I think a "Contract for services" in the UK. There was no written agreement.

You created the software, and so you owned the copyright when it was created, and you made no agreement to sell the copyright, as opposed to the right to use the software (a license), to your client. (In the US an agreement to sell a copyright must be in writing, I am not sure if that is true in the UK.)

The right to create modified versions, known as "derivative works" is part of the copyright, so you have that right.

If I am correct you therefore have the right to make modified versions of the software, and sell them to others.

It would be good practice to develop a standard contract for your work in future, spelling out just what rights you are and are not selling to your client. It could also spell out your payment terms, by hour, or by job or whatever you use, and whether the client has the right to reject or insist on changes to the finished software. This would avoid problems by having everyone understand the deal in the same way. There are various published books that give templates for such agreements.

I am not a lawyer, and there are no doubt details you have not included in this question. You would be wise, and safer, to consult a lawyer with knowledge of copyright in the UK, to make sure that your plan has no snags.