I have been in several official Monopoly tournaments, so I have had a little experience in these things. According to the rules, once you have all of the properties of a color group, you may immediately (or later, if you like) build houses and/or hotels on the properties. You may build as many or as few houses as you like or have the money for; you are not restricted to one round of houses per turn. BUT ... you must build your houses evenly. This means you cannot build a second house on a property until each of the properties in that color group has one house. You cannot build a third house on a property until all of the properties in that color group have two houses. And so on. If you wish to build only one house on a single property for which you own all of the properties in its color group, that is okay according to the rules. Each house is technically built separately, so the even-building check is made after building each and every house.
To build a hotel on a property, you must first have four houses in place on every property in its color group. You cannot simply assume virtual houses and pay a lump sum for the hotel upgrade. If a lot of houses have been bought by other players, there may not be enough houses for you to upgrade your color group to hotels. (There are supposed to be only 32 houses and 12 hotels in the game.) If the bank runs out of houses before you can build four houses on each property, you are out of luck. In fact I use this strategy quite a lot in Monopoly. I tend to buy up to four houses per monopolized property I own and refrain from upgrading to hotels so no one else can build anything. Although this is considered poor form by some players, it is a perfectly legal and often excellent strategy. - LJS