Congrats! The progress seems remarkably fast. Just a couple of things off the top of my head, hope they help you in some way.
These are bits and pieces I picked up along the way and I'm by no means an expert.
1. Seems like your neighbor is also doing the exact extension. What about the other side? Do keep pictures of not only your property but the before & after of your neighbors. This will be useful if you get into any legal trouble with them. Do try to be on good terms with them though.

2. If you are building a new bathroom, do ensure that the floor is sufficiently sloped and water does not pool. And don't skimp on waterproofing. Ask your contractor to do a ponding test.
3. Will you be installing ventilating fans? They should vent outside your house, not into the ceiling space.
4. Is your manhole easily accessible?
5. Since you like spotlights, do you want to consider making several of them dimmable? e.g. in the TV den, or in the bedroom.

6. For air conditioning: minimize the distance between the compressor and indoor blower unit.
7. Do allow high-powered appliances to be on their own circuit rather than looped. On a similar note, invest in better quality fusebox components (the MCB, ELCB? etc - not too well-versed). Similar note #2 - ensure grounding is proper or else your power will be tripping everytime it rains.
8. You may want to work with your electrician on the placement and order of switches, to avoid weird / unnatural placement. Certain areas e.g. staircase, bedsides switches (if any) may require 2 (or maybe 3)-way switches.
9. Also, do consider installing timer switches.
10. Any solar panel involved? Which areas or basins do you want hot water to be available? Your plumber will need to know.
10a. Do check if the water pressure is sufficient especially upstairs, otherwise you may want to consult your plumber. Otherwise, can consider installing a water pump.
10b. Outdoor water filters (if any) - got space to put? Indoor water filters (if any) - requires power? May want to extend a hidden power outlet nearby.
11. Will you be installing high cabinets, pendant lighting, chandeliers etc? Do ensure they are out of reach from your fan blades (especially when the cabinet doors are ajar)

12. Polytank - may be able to be placed / hidden inside the ceilings, which saves you from having to build a concrete slab above your second floor (if placing on the roof) and presenting more water leaking problems in the future. Not too sure about the cleanliness... our parents survived just fine on decades-old polytank water right

anyway I don't know. I went for SS.
13. Are you on 3-phase? Do try to balance the load across the 3 phases. Your electrician will know how to do this.
14. Council approval and the assessment of a qualified architect & engineer is also something I would recommend. A few forumers have expressed the possibility of cracks forming in the medium term. Now would be the time to query your contractor and/or architect. I think my extension may also face this problem.... as the footing looks similar.

(My renovation was a 2-year, unending nightmare)
15. Do note that to complete your renovation from a legal standpoint, you need to have the CCC issued by the council after all works are done. I hope the price you paid includes this.
Hope it helps.
This post has been edited by phoenixxx: Jun 1 2019, 12:38 AM