Health and safety law that applies to building and construction work
The Act that always applies to this kind of work is the Health and Safety at Work etc Act 1974. It essentially demands that the workplace is safe for everybody who is working there and anyone who might be affected by the work done.
If you are a domestic Client, it is unlikely you have to worry too much about this, although the people who are doing the work for you will need to consider it all the time.
There are also a set of general Regulations called the Management of Health and Safety at Work Regulations 1999. These say that the risks to safety and health of work activities must be thought out and controlled properly to make the workplace safe. Some employers with 5 or more employees will have to write these assessments down so that the information can be given to the people who need it.
There are also some construction specific regulations as follows:
The first of these says that where construction work is going on every one should wear a hard hat unless the person in charge decides it is safe not to do so. If that decision is taken, the responsibility for the consequences rests on the person in charge who took the decision.
The second set of regulations, referred to as CDM, are about planning, resourcing and passing information on. They demand that any designer considers how to design out risk and that every company appointed to the task is competent. If you do any design you will need to know more about these regulations.
CDM also puts heavy duties on Clients but this does not apply to domestic clients. Be careful though, if you have more than one property and are working on a property to let it out or sell it on you may be classified as a Developer, in which case the Client's duties will apply and you should find out more, though you architect, engineer, quantity surveyor or other designer should be telling you what you must do.
The third set of regulations is about how the actual process of construction must be carried out safely and sets standards for excavations, work at height, scaffolding, work off ladders etc. It really applies to those who are doing the work.
If you want to know more the best summary is a book called Health and Safety in Construction published by HSE Books. It costs £7.95, is referred to as HS(G) 150 [Health and Safety General series No 150] and can be ordered from any good bookseller or direct from HSE Books.
Another useful reference point is the Working Well Together campaign website
Building extensions or alterations to property
If you own property that you wish to extend or alter and employ others to do the work it is very likely that you will not need to worry too much about being prosecuted for a breach of the criminal law.
What is much more likely to be important to think about is the civil law of tort.
As an occupier of premises, that is the person in overall control of a building and its environs, you have a duty to any one who visits those premises either lawfully or as a trespasser.
When you have building work going on the area is almost certainly going to change from normal and is going to be more dangerous than usual. It is likely that your visitors will not be familiar with building sites so you will have to be extra careful for them.
Things to look out for are holes or any excavation, which should have a secure barrier around, material falling from height, which should not be able to harm people below and dust and fumes from work activities.
Remember, you have a contract with your builder so you can give him instructions because of that. Be careful though not to get too involved as you may end up having to pay for additional work or being counted as jointly liable with the builder if you restrict his choices too much.
Generally you should expect the work area and its surroundings to be tidy and separate from the walkways you and you family and friends need to use.