Your architect can do an opinion of compliance with building regulations and with planning permission.
If the issue is that you are not planning on meeting with this regulations, it is obviously going to cause an issue.
Building costs are absolutely crazy at the moment and even builders themselves are saying they are finding it hard to get contractors and their costs have shot up. Unless you know what you are doing and have good contacts, I would caution about doing it yourself. If you have an electrician or plumber booked in and he gets a call from a builder to work on the same day as he's booked in with you, who do you think he'll go with? The one off job or the guy who will give him a load of work? It will take you a lot longer and a lot more heart ache doing it by yourself.