Not all are complicated, because some performance takes only a few minutes. This may be the installation of a kitchen tap, especially if the owners of the house they purchased a modern faucet that is simple to install.
Connecting the kitchen dishwasher
The kitchens are made different hydraulic works.
Also repair this faucet should not cause major problems, although it may be that even the replacement of seals in a modern tap will only be possible during the visit plumber.
In recent years the standard of cooking was also the presence of a dishwasher, which helps housewife doing everyday activities related to washing dishes.
Sometimes, however, when connecting the dishwasher must be present plumber.
Wikipedia - plumbing standards and regulations
Much of the plumbing work in populated areas is regulated by government or quasi-government agencies due to the direct impact on the public's health, safety, and welfare.
Plumbing installation and repair work on residences and other buildings generally must be done according to plumbing and building codes to protect the inhabitants of the buildings and to ensure safe, quality construction to future buyers.
If permits are required for work, plumbing contractors typically secure them from the authorities on behalf of home or building owners.
In the United Kingdom the professional body is the Chartered Institute of Plumbing and Heating Engineering (educational charity status) and it is true that the trade still remains virtually ungoverned; there are no systems in place to monitor or control the activities of unqualified plumbers or those home owners who choose to undertake installation and maintenance works themselves, despite the health and safety issues which arise from such works when they are undertaken incorrectly; see Health Aspects of Plumbing (HAP) published jointly by the World Health Organization (WHO) and the World Plumbing Council (WPC). WPC has subsequently appointed a representative to the World Health Organization to take forward various projects related to Health Aspects of Plumbing.Źródło: https://en.wikipedia.org/wiki/Plumbing
Encyclopedical explanation of the valves
A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways.
Valves are technically fittings, but are usually discussed as a separate category.
In an open valve, fluid flows in a direction from higher pressure to lower pressure.
The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction.
This is called a check valve, as it prevents or "checks" the flow in one direction.
Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.