Do we need an achitect for a domestic job. See below

Local Authorities require plans that are compliant with the Local Planning Guidlines and Building Regulations, they can therefore be created by anyone, provided the plans and details are compliant. Engaging an Architect provides the Assurance of a minimum level of competency capable of compliance.

Yes you will need an architect to prepare drawings for planning (if needed) and to submit building regulations for approval.

All the above answers are useful, however if permitted development rights are available then go to the Planning portal website and look at the options available to you. In a nutshell a semi detached building can have a single storey extension that projects into the garden no more than 3m and the width can be across the entire width of the property or a detached property can project into the garden by 4m. There are height restrictions and boundary conditions but these are available on the above site. You would still need to hire an architect or technician to produce drawings for building regulation approval but save on the planning fee

You need someone with experience and knowledge to draw what you want to build. It is best if this person is either an architect, architectural designer or technician. They should all hold appropriate levels of insurance for the size of projects they work on and provide you with references and previous works if asked to do so. If its a small single storey rear extension under 3m from the back of your property, you won't have to get planning usually (unless in a conservation area or if the building is listed). However, every project will need detailed drawings for a building control application. Some councils are now adding upto 20% more to the fee of a building notice (the application without drawings) so providing drawings will be more cost effective and you are unlikely to get an experienced builder adding more to your build cost they quoted.