England and Wales have different rules, under Part P you need to follow laid down codes of practice, does not need to be BS7671 but since that is the British one most would follow that code.
Also certain circuits require notification, in England there is a list of what needs notifying, in Wales there is a list of what does not require notifying. Wales is more restrictive to England, Scotland also has similar rules but not called Part P.
It is part of building regulations so boats and caravans are not buildings so are not covered. It also only applies to domestic, and to work out if domestic or not it states if the garage gets it's supply from the house then it is included in the domestic group.
In England you can't add a circuit, however it does not seem to use BS7671 definitions as that would mean adding a FCU forms a circuit yet it seems with English Part P only a circuit from the consumer unit is counted.
The rules are really hard to define, a consumer unit is a type tested distribution unit, where an ordinary person is in control you should only use type tested distribution units, however if you put anything in the consumer unit not agreed as permitted by the manufacturer, then it's no longer a consumer unit. There are arguments that adding something to a consumer unit to an existing MCB is not a new circuit so permitted.
So it is a typical British law, let the courts decide. As an example on the old TV licence it said apparatus for receiving broadcast TV, well that is the aerial however painting the aerial black and white does not mean you only need a black and white licence.
So you can do what ever you like, until some thing goes wrong, only then will anyone look at what you have done. So to pay the LABC their fee, means in real terms if it goes wrong you don't carry the can.
Both England and Wales you can do what you propose without paying a fee, it's not a new circuit, it's not in bathroom or if in Wales also garden, and kitchen, as well. However the new sockets must comply with BS7671 so need RCD protection and cables protected from likely damage.
After you have finished the physical work, you need to inspect and test and fill in the minor works certificate blanks can be down loaded from the IET web site free of charge, you with not need a completion or compliance certificate.
And if some one is injured the HSE can request to see the minor works certificate where you sign to say you have the skill to do the work.
However if nothing goes wrong, and the home is owner occupied, no one is likely to ask to see the minor works certificate. If rented then completely different, specially in Scotland, then the person testing has to have training and even though I am an electrician I would not sign the electrical installation condition report for a house owned by me, mainly as no insurance, but also don't feel it is right to sign off my own house even if technically allowed.
So being blunt unless you have all the test equipment and training you should not do any electrical work, however we all do, and with just a simple EZ150 plug in tester the likely hood of you making an error undetected are low. In other words use some common sense.
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