I purchased a home in Lutz, Fl (outside of Tampa) about 3 years ago. I
noticed a while ago that the space between my neighbors house and mine
(side of the house) was always somewhat soggy. No matter what the
weather was like, it was always soggy. It was always on one side of
the house. What could be causing it and what type of tradesman should
I call? The city? The sprinkler system people? A plumber? What can I
expect to pay for such a problem?

Here are a few ideas to get you started.

WHAT SHOULD I DO?

Formal assessment of subsidence damage is neither simple nor certain
and is normally a matter for an experienced engineer. However, there
are a number of things you can check:

 Does water collect next to the house? Is the lawn growth more
vigorous in some parts than in others? Is the ground always soggy in
particular areas? These factors indicate potential problems because of
uneven moisture in the soil next to the house.
   
 Have you or your neighbours made any changes to your gardens in the
last couple of years? For example have you planted or cut down trees
or large shrubs, put in paving or paths near the house, or altered
drains near the house or installed timed sprinkler systems? (This may
affect soil moisture near the house. Remember, the root system of
trees can cause damage to the foundations of a structure some distance
away by removing moisture from the ground - refer to the 'Other
Factors' section earlier in this Fact Sheet).
   
 Is your plumbing and drainage in good order? Check for external
leaking taps, dripping hot water system overflows and toilet cisterns,
and for signs of broken pipes. If you're not sure get your plumber to
check for leaks.
   
 Is your sub-soil clay and does it feel like plasticine when moist?
Can you see cracks in the ground when the soil is dry? Are fence posts
and power poles around the house out of plumb? (These signs may
indicate the presence of reactive clays which could contribute to
subsidence in extreme conditions or if the site is not well
maintained. Ensure an even moisture profile is maintained to perimeter
of your home).
   
 Have you or your neighbours built a retaining wall near your house?
(A rule of thumb is that retaining walls can cause damage when they're
closer to your house than about one and a half times the retained
height.) Retaining walls over 1 metre in height require Local
Government approval and must be designed by a Registered Professional
Engineer and should take into account slab/footing design and soil
profile.
   
 If you have the soil test report and footing design for your house
you should check the advice on site maintenance contained in those
documents.

http://www.bsa.qld.gov.au/consumer/bsakit/subsidence.htm