wiring extension

hi all sparkys i have a ? i have just built an extension on house with diningroom, bedroom ,downstairs loo.i have run 1.5mm for lighting circuit using junction boxes,going to connect new circuit to exsisting up stairs lighting circuit as upstairs only has 3 lights on it low energy too. havnnt looked yet but if upstairs circuit is in 1mm can i connect 1.5mm cable to it have 4 lights in exstension plus 60 watt shaver socket and 20 watt fan. exsisting upstairs cable is under loft insulation which makes difference hey . Also i have an old 15 amp ,2.5mm immersion heater circuit no longer in use i was going to use as a radial circuit for a few sockets is the maxium 35 m length i have rewirable fuses . id like to make it a ring at a later date , and have the new lighting circuit separate when i change consumer ,having probs finding consumer less than 200mm high to fit below cupbard all advise appreciated

The 1.5mm² should be fine - its protected by the fuse / MCB at the CU, provided this is 5 / 6 amp. Check the total load of the lighting circuit wont exceed the rating of the fuse, or else you wont be able to have all lights on that circuit on at once. Not a big problem though. As the cable is in insulation, 1.5mm² should be used anyway ideally, but may not have been done. Can you possibly pull the whole cable above the insulation while you are about in that area? And clip it to the top of the joists.

Do you know about part p of the building regs? Are LABC coming to inspect your work? (electrical and building)

You can convert the immersion circuit, but why not do it right first time, to stop you having to chase out plasterwork afterwards? Just run another cable back to the CU.

CU changes should only be done by very competent DIYers, which by your post it sounds like you are not, no offence. Best to get an electrician in for this. Why have the lighting circuit seperate? I see no advantage to this, unless you are RCD protecting the whole new CU?

cheers thanks for advice crafty yes building control have to come for extension. i've read about part p it says adding new lights and sockets to exsisting circuits are exempt as long as they are not in bathroom kitchen or outside . will the building control want to see a certificate?. i need to get new consumer so most probably do all the dirty work and get sparky to connect all up and fit new consumer unit . i found part p rules regs vagigue . any way thanks for the advice much appreciated