Wimbledon Architecture Home Extension

Architects Fees & planning an architectural project

How much does an architect cost for an extension?

When planning an extension or any architectural project, one of the first things you’ll likely consider are architects fees, it’s only natural! Questions such as what’s included with the fee, and do you need to appoint an architect for the entire project duration crop up frequently.

Architects’ fees largely depend on the nature and size of the project and the circumstances of the appointment. They can vary significantly; since the Royal Institute of British Architects (RIBA) abolished their fee scales back in 2009. As an approximate rule of thumb, it might be useful to expect total architects fees to be around 8 – 15 % of the overall construction cost for the project.


Most architects will break down their fees to reflect 6 predefined stages of a project, which have been established by the RIBA known as the ‘RIBA Plan of Work’. These stages should help you to get a clear picture of exactly what it is you are getting for each portion of the fee. They also allow you to clearly understand what payment is required to get to each critical stage of the project. This is useful if you do not intent to go through all project stages, for example you plan to achieve planning permission only and then sell your site / home.

If an architect has sent you a fee proposal that is difficult to decipher, we recommend asking them to structure it using the 6 RIBA Plan of Work project stages mentioned above; and to explain exactly what they will provide you for each stage. These stages are outlined below:

  1. Design Brief
  2. Concept Design
  3. Planning Design ( It is at the end of this stage that a Planning Application will be submitted )
  4. Technical Design ( It is at the end of this stage that a Building Control Application will be submitted )
  5. Construction Phase
  6. Project Completion

The following diagram presents a typical percentage breakdown of architect’s fees by RIBA project stage. The 6 stages can usually be simplified in to 3 key phases for a extension or newbuild project. Planning Permission (Stages 1-3), Building Regulations Approval (Stage 4) and Construction / Final Handover (Stages 5-6).

Architects Fees ( By RIBA Stage )
1 - 3 30%
4 40%
5 - 6 30%

To get up to Planning ( Stage 3 ) is usually around 30% of the overall fee. Building Regulations ( Stage 4 ) tends to be around 40% with the final 30% for Construction & Handover ( Stages 5-6 ).

Architects fees depend largely on the complexity and scope of work involved. Whether you are wanting to learn how much an architect would cost for an extension, or any other construction project, It is important to try to be as descriptive as possible when discussing your project brief. This will ensure your quoted fees are kept to a minimum.

 Taking a standard single storey extension as an example, based on an overall average fee of 12% and a project construction value of £100,000 you could expect architects fees to be approximately £3,600 for Planning, £3,600 for Building Regulations and £4,800 for Construction.

CDJM Architects Stage 4 Section Drawing