If you have ever considered buying an old house with the idea to fix it up, then one of the first things you probably thought of was ... level the house first, then fix it up.
Why isn't it level?
Before you do any leveling, you need to know why it isn't level now. There are a number of reasons why it may not be level.
Begin this process by inspecting the foundation itself. Does it have a foundation, or was it build on post and piers? Check to see if beams and floor joists were set on blocks of wood, or large rocks, or simple set directly on the ground.
Next, check ground clearance. Clearance should be at least 12" from the floor joist to the ground (although lenders may require more clearance than this).
Also check for good ventilation under the house and the condition of the wood in the underfloor section of the house. A ground cover of black 6-mil visqueen is a good idea.
After determining that the underfloor part of the house is sound, you need to determine why the house isn't level. Depending on severity, the house could have settled due to underground water, earth tremors, or just natural soil erosion.
You may need to correct conditions that caused the house to go out of level. Underground water and erosion must be dealt with, or the house will never be levelable.
Even after determining and correcting the cause, it still may not be a good idea to try to level the house. If the settling was caused by broken or rotten lumber, and leveling is minor, then leveling could be done. However, if the house is severely out of plumb, correcting the problem may cause even larger problems.
Keep in mind that a house will generally settle slowly, over many decades or even centuries. During this time, there could have been many repairs and remodeling done to the structure which may have sealed in the crooked walls and floors. Even if done slowly, the resulting correction could cause major cracks in the walls, shift doors out of plumb so they won't open, and even possibly crack the windows.
Over the years, wood can harden become as hard as rock. In making repairs to a barn built in the 1800's, the wood was so hard that we couldn't nail new wood to old wood using a nail gun! On another remodel, we took ou a wall to open up the inside of a home built in 1900. The house had settled and when we tried to level the 2nd floor, we had to replace floor joists because thay had hardened into their warped postion.
These are just some things to consider when considering purchasing an old home. You might be wise to treat that dip in the floor or tilt in the wall as just "part of the character of that old place" and leave it alone.