The first step is to create drawings of the existing conditions. Sometimes these are available but most times it requires field measuring. For an average house of 2,000 square feet or less I'll spend about four hours measuring on site and about four hours entering the information into my drawing program. Then I'll save a copy of the drawing (CAD) file and be ready to start the first design in plan view. When I measure on site I take off the dimensions of the rooms and size and locate the doors and windows. Then I look in the crawl space to determine floor framing (if it is a raised floor) and look in the attic to see how the roof is framed. Then I take off the roof slopes and the ceiling heights. I generally come with a ladder and will go on the roof if is helpful ( and safe). I also look at the foundation and try to determine the foundation type. Next I will locate the house on the site by measuring off the curb and side and rear fence lines. I will also locate the electric service, gas meter and review the water heater and furnace. Finally I will take a lot of digital pictures of the interior, exterior and adjacent properties. If the site is sloping I may also take some rough heights of ground levels in the proposed development area. Most measuring I do with a laser but some measurements, like window sizes, are easier to measure with a good old fashioned tape. Any drawing that the client may have is helpful. If there are actual construction drawings I still like to verify critical dimensions as there are often changes and variances from the drawings. Often I'm asked why I need to measure the whole house when the project is in just one area. The answer, based on long experience, is that designs that are developed without looking at the whole plan together are often ill considered.