I worked in construction in the UK for a good few years and hold some qualifications in this area so maybe I can add some meat to this.

1) Timber just missing a stamp...So timber used for structural work is stress graded. That is it is either bent by a machine which checked how far it deflected or it is visually graded by a qualified grader. Someone who mills their own timber could have employed a grader to do this, and it is not really a barrier. However construction timber must also be seasoned which adds strength and removes moisture which if trapped in can cause fungal rot. In the UK and for the timber we import from Canada (CLS Canadian Lumber Standard) this is normally achieved with a kiln drying process, which conveniently kills insects living in it too. It is hard to see how he achieved that. Timber in the UK used for roof members has to also be treated with wood preserver. It is rather more than just a stamp! I will add that a home mill comes with a lot of overhead and it is not clear that it wouldn't be cheaper to buy the timber, especially when you consider the better dimensional tolerance of machine regularized timber.

2) The building code is relatively strict in the UK but poorly enforced. This in the main is due to their not being enough inspections of the finished structure behind the drywall (plasterboard as we call it), especially when it comes to insulation/vapour barriers/noggins/metal straps etc. Some of the private building inspector schemes seem to be far to chummy with the builders.

3) When building something like a roof truss 'the way they used to in the 1960's' ie, not from approved plans, in general running the designs past a structural engineer will get you approval. This will cost you, but that is the cost of varying from the tried and tested. Same as if Geohot wants to build a self driving car he is going to have to demonstrate the safety himself.

4) There should be no exception for self build unless there is a covenant that the building is pulled down upon you leaving it. I would also suggest that you should have to post a bond to pay for the cleanup work. Otherwise the naive builder could be passing on a death trap. Likewise, the buildings around you rely on you doing your bit for fire prevention. The Fire-fighters who may have to go in and save people from your house should be able to rely that the fire barriers are properly in place etc...

5) Buildings are much more complex in the way they live and breath and the ergonomics they offer than what a lot of people give them credit for. You may curse that there is a regulation for the height of a light switch...until you are in a wheelchair. Keeping an insulated house dry, especially at the interface of cold warm air, is much more complex than grandpa's old log cabin.

6) The house I rent was 'done up' by DIY buy-to-let guy. He didn't see fit to run his electrical cables straight up or down from sockets so I can't even hang a picture for fear of nailing a cable. The drains block from the shower and sink as their is insufficient fall. The stair spindles are over 100mm apart and shouldn't be (this rule stops children from being strangled because they can't fit their heads in the gap). All of the doors have been hung without leading edges, so they catch when you shut them...All this stuff is stuff DIYers are not going to pick up from youtube. Personally I think the rules should be tougher and DIYers shoudl stick to painting and building sheds