We have raked ceilings in our new split level extension that follows the fall of the land (and in the rooms that were part of the skillion sections of the old cottage) and I love the sense of space they give and also the fact that the inside reflects what is happening on the outside of the house. We have used ceiling boards (painted white – I didn't want to feel like I was living in a cigar box) both in the old cottage and in the new section and they certainly add an enormous amount of visual interest to the rooms as well. However, I endorse the comments about the added trickiness of light fittings. Pendants with rods are out and pay particular attention to the little rose/canopy thingies that connect pendants to the ceiling – make sure that they sit flush with the ceiling and don't have weird unsightly bits that you can suddenly see because of the angled ceiling. We have successfully used LED downlights with a recessed diffuser and dimmer switches to make sure there is no glare but if the raked angle isn't too extreme they are a definite possibility.

I work in tiles and the normal splashback height is 600 mm. This is because overhead cupboards are usually this height above the bench top. In your case you don't have this so can do anything you like. If you tile to low 200mm etc the wall gets damaged - paint doesn't wear like tiles. There are many materials you can use as a splash back but most aren't durable eg Perspex. Glass and tiles have the edge. Textured splashes are in hence the hand made tiles. If grout annoys you use large tiles you can get them up to 1200mm long these days with rectified edges you only have 1.5mm gap. Use sealer on the grout, they are now easy clean. If you have the money tile to the bulkhead a much more luxurious look. Maybe in a charcoal. I don't know how these gaps are getting so dirty. I use spray and wipe with a toothbrush occasionally and it takes little time and always clean.

totally depends on the context and what exactly the applicable planning policy is/reads and is endeavouring to control/protect... eg is it view sharing? heritage?.......best chance for you to obtain approval beyond a specified limit or trigger (assuming it is a reasonable proposal that doesn't adversely impact neighbouring property) is to work with a design or planning professional who is experienced in negotiating the planning process, and is able to competently prepare the proposal for submission to relevant council (via a formal planning application process) with all the necessary supporting documentation/diagrams/reports etc

Yes, every rooms will have ceiling fans and air conditioners. ( due to hot weather) My concern is that the built-in cabinet may not look very nice if it’s made too high. I wanted to extend it up to the ceiling to make it look neater and prevent dust collection. However, my contractor mentioned that if the living/dining room ceiling is high and the bedrooms’ ceilings are low, it might look odd when entering the rooms due to the different height levels. Is that true?