I am an architect and I do a lot of small jobs like this.
We’ve found the best thing to have a very clear and specific appointment document, that way the client is aware from the word go about what they’ll get. We can also recommend forms of appointment for builders to make sure the relationship between all parties is clearly defined. Managing a client’s expectations is a difficult part of the job as many people have little of no experience of the industry and need guidance through each and every step. It may seem expensive, but you very often get what you pay for.
Yes, people fall out and disagreements arise, that’s why it is also important that any appointment document lays out a framework for dealing with these situations. RIBA have a lot of guidance on their website (so they’re good for something). Alternatively drop me a PM and I’ll do my best to help you out.
A good architect can help guide you through the whole process from start to finish and will deliver the project you want. A good builder is key to this process. Find both and you’re laughing.