We've just finished ours. Building officer came out just before Christmas to do his final inspection and issue our certificate.
Ours was a long single car garage for 1960's, double depth but single width. We converted it into a study/ home office to the front and large utility space to the back flowing off the kitchen to the back.
We checked with our local planning department and they confirmed that due to laws on development softening, even though we were changing the front face of the building, no planning was required.
They have issued us with a certificate of lawfulness (I can't recall exactly the name) so we can put it in with our selling pack when the time comes to sell.
We did need building control. We didn't need technical drawings as ours was a pretty straightforward single story conversion.
The building control officer was assigned and was helpful. He had to be present at various stages of the conversion to tick various boxes. He was happy to have a photo trail of our work and pop in when he needed.
Initially we got quotes for builders to convert it but they were coming out at £14,000 more that our estimate by the time they added VAT and profit.
So in the end we employed a bricklayer directly and he did the bricking up and fitted doors and windows (these were signed off by building control, you don't need specialist fitters for a conversation). We also got in a local plasterer and an electrician in as these skills are beyond me. The electrician completed his certificate and this also has to go in your seller pack.
We cracked on with the rest.
We now have a lovely warm office, huge utility space, and a fibreglass roof that should last 50 years. It came in at a tiny £13,700!
I'm pleased and would do it again. The thing that really helped was once building control realise you are doing it mainly yourself they are very much more hands on. We were carefully guided through all the stages as they need to inspect them.