Once we had agreed on the purpose and layout of the extension it was time to hire an architect to draw up the plans to be submitted for planning approval by the council (i.e. make a planning application). This was much more expensive than we realised, even though we had come up with the main plan ourselves.
Update: July 2015 – now that I am planning a loft conversion, I have discovered that it is possible to get the cost of the plans down a lot – I paid £775 for my loft plans and this is all I need! More on this soon.
Update: April 2018 – I moved to a bungalow and am now building an extension. I did all the designs myself, for free! Find out how I designed and planned my bungalow extension here.
The Planning Application Stage
The architect charged around £300 for the initial draft, then another £300 before the plans were sent for review by planning. We had to pay around £170 direct to the council ourselves too. After a couple of months he plans were fully approved. This process took much longer than expected.
The next stage is the building regulations. For this new plans are drawn up, and these detail the construction of the extension. These plans are more detailed. This stage also cost around £600 (£300 initially and then another £300 when ready to go to the Building Regulations office at the council). This process also took much longer than expected. Several changes were requested by the council and made (although we did not get a report of these changes).
Note that the initial plans had a smaller French door and windows, 4 roof lights, no wall in the utility and the kitchen is still along one wall. The main issue at this stage is the size and shape of the proposed extension.
The Building Control Plans
The building control plans specified everything from the depth of the foundations (it gave a minimum and maximum depth needed, which is then decided by the building inspector on the first visit) as well as the type of materials needed for the construction of the new extension.
Once approval was given by building regulations we could then start to get some accurate building quotes. Until now builders could only give a rough estimate of the cost to build the extension as they did not know the type and amount of materials. However, as soon as building control have approved the plans it is all systems go, and the builders provided more accurate quotes for the job, and we then picked our favourite builder.
The architect provided construction drawings which are the same as the plans for building control but with more measurements included. As well as the drawing plans there is also a lengthy document which details every part of the construction, specifying strength and load bearing of walls etc.
Note that on the final plan the utility wall is added.