That there's a lot of extra costs and council fees, cross over etc.
Spend the money on things you can't change, house size, layout, ceilings etc. light fittings can be done later for much cheaper!
You need more power points than you think. Consider two way light switches for long halls.
Think about what you want and how you will use the house rather than resale.
My favourite change we made in our house is a double shower. So glad we got that instead of a corner spa.