In the UK you need a design approval and a production approval (£££ and time). The design has to be approved (£££ and time) and hoops jumped through before it can be flown. Once you have flown it any changes have to be redesigned and re-approved (you can't just wheel it back in to the hangar, tinker, then wheel it back out again and see if that worked).
I know the system, I'm a CAA design signatory,a BMAA Senior Inspector, and a BMAA and LAA Test Pilot. The people who use these excuses not to innovate are generally those without the skills to generate good products anyhow.
The costs to be paid through BMAA or LAA are pretty trivial, no DOA/POA is required, and ultimately it comes down to the time and effort required of the designer to get it right.
At the certified end it is usually different in that the sheer costs of getting kit certified in Europe are far higher than in the USA with its very elegant DER system, but this thread was about homebuilts. But even then, I recently looked to two changes to my vintage / UK-reg / CofA aeroplane.
(1) Fitting an uncertified oil cooler; this took my effort, purchase of the parts, and once I'd sumbitted a report to CAA, a charge of £61.
(2) Switching from AVGAS to 91UL; this I managed to do on a paperwork exercise and has cost nothing.
Ultimately what I needed here was to be competent, not any deregulation.
Designing an aeroplane is much harder, and requires different skills, to flying one. But, many people do not recognise that, and whilst they accept that they need formal training and assessment to fly, somehow don't believe that this should apply to designing and building.
G