In my experience Ultra Luxury Residential Homes or Ultra Luxury Real Estate share common features within this five areas :
Location , Design & Quality , Size , Amenities & Equipment , Price.
Location: Ultra Luxury Homes have over the top locations, meaning privileged views of the ocean, lakes, rivers, canals, golf courses , valleys, other landscapes or beautiful city skylines in case of high rise condominiums.
Design and Quality: The attention to detail has to be present on the architecture and general design of the property as well as in mostly every interior room.
Size: Here we are talking about several thousand square feet or several hundred square meters of indoor areas. In my market, South East Florida ( Fort Lauderdale, Boca Raton, Palm Beach or Miami ) Ultra Luxury Homes as a bottom line start at 5 to 6 bedrooms and baths and in case of Condominiums at least at 3 to 4 bedrooms and 4 to 5 baths. Add of course plenty of open non air conditioned areas or balconies / terraces.
Amenities and Equipment/Technology: Garage space for all the toys, infinity pools, wet rooms, 10 to 20+ guest capacity movie theater, sport courts and gyms, game rooms, outside and inside bars or summer kitchens, wine cellars and dozens more of possible amenities are not uncommon on Ultra Luxury Homes.
On the equipment side expect top brands of appliances, integrated smart controls that work from your phone and other features.
Price: This could be the most relative of all Ultra Luxury Property characteristics . Different areas or countries have different land prices and cost of building materials and labor, but most likely anywhere you go prices will start from six zeros and up.
In South East Florida my area of expertise most waterfront homes that can be considered Ultra Luxury start near 5 million dollars while the Condominiums will start at over 3 million. Again is relative, some superb or really large residential lots with access to the ocean and Intracoastal Canal to dock a mega yacht like the ones you find on The Hillsboro Mile will ask only for the lot between 5 to 10 million dollars so do some math an imagine at what level a fully finished home starts.