What is the cost of an extension?


We couldn't believe it when we found out how much an extension costs when the first quote came in via email from the contractors.

Opening that email was one of those ‘slow motion’ moments, one where you literally do a double-take and rub your eyes with your fists like a cartoon character and take another look. Woah…

Considering an extension

To set the scene a little, the house that we bought was an average sized 1930s semi-detached and although it was liveable in, it was certainly in need of some serious renovation work.

Like many 1930’s houses, the original layout (especially on the ground floor), wasn’t cracked up to deal with many of our 21st century living standards.

floor plan.jpeg

The layout of the area on the ground floor (pictured above left) involved:

  • A small galley kitchen and larger dining room, separated with a ‘convenient’ and ‘stylish’ serving hatch

  • A lean-to extension (sunroom) accessed via both a utilitarian patio door in the dining room and a back door in the galley kitchen

  • A small WC which was more like an outhouse than an actual WC; freezing cold, full of spiders and filth.

It all needed some very serious thought.

So other than bringing the rest of the house up to the 21st century, the main area we wanted to focus on was a knock-through kitchen diner, and on the table was a proper extension with a roof (rather than the existing sunroom with a plastic roof) .

The back of our house before we moved in

The back of our house before we moved in

Costs of an extension – Our requirements

Without an awful lot of thought, we brought an architect on board to understand what we could do with the space, both from a structural and a layout point of view and the initial idea was to create an extension. The original plans involved:

  • Moving the toilet from the back of the house to next to the stairs

  • Knocking both of the supporting walls down between:

    1. The existing kitchen and the dining room and;

    2. The rear wall into the existing sunroom

  • There would be a pillar installed to support both walls and two RSJ steel beams installed.

  • A roof would be constructed with possible skylights to improve the light.

The equity that we had gained from the sale of our last house was about 70K – we thought this would be more than enough to renovate the house to decent level, including a new kitchen extension.

We were very, very wrong.

OUR Architects original plans for our extension

OUR Architects original plans for our extension

The costs of an extension

Single Storey Rear Extension - what our quote included

After we got our plans drawn up by the architect we obtained a quote from a contract. Here is the original extension quote itemised:

  • Remove existing kitchen and clear from site leaving a temporary sink 
  • Remove existing ground floor WC and clear from site 
  • Walls to be demolished to create new openings clearing all waste from site 
  • Remove plasterboard from existing ceilings to facilitate the installation of steel work and creation of new openings 
  • Supply and fit 2 x steel beams and pad stones to prepared openings as per drawings and consultations 
  • Box in steels using fire-line boards and treated timber 
  • Excavate down then underpin existing foundations as per structural engineers drawings 
  • Carefully remove existing conservatory and necessary brick work and clear from site 
  • Cut out and install a new cavity tray with weep holes into existing cavity above new extension 
  • Build cavity wall to existing brickwork as per drawings matching bricks where possible with openings for patio doors 
  • Construct roof with treated timbers and firings and 18mm WPB plywood and 150mm celotex insulation 
  • Supply and fit single ply roof system to prepared roof 
  • Supply and fit two Glazed fixed roof lights as per specification 
  • Supply and fit two sets of patio doors in Aluminum as per drawings with additional side window and new kitchen window 
  • Supply and fit glass Juliette balcony across one set of patio doors 
  • Erect stud partitions for ground Floor WC 
  • First fix all plumbing and electrical work to ground floor as per specification 
  • Plasterboard all necessary walls and ceilings then plaster providing a smooth finish
  • Install provided Pan and cistern, under floor heating, flooring and tiles 
  • Install new skirting, architraves and doors to the ground floor as required 
  • Run new heating pipes then install supplied radiator 
  • Run new gas supply for cooker 
  • Supply all Materials and Undertake all works as per specification, to comply with local building regulations 
  • Basic lighting and electrics (up to 2100+vat)
  • All waste material to be cleared on completion

    Total = £51,250.00 + VAT

Add onto the above the supply and fit of kitchen and appliances, furniture etc and you’re looking at a total bill of around £80K. We would even be doing all of the DIY…

Eventually we decided to change our plans for an extension; a decision that we didn’t take lightly and if you’re interested what we decided on you can read all about this here.

The quick version however is that we decided against doing an extension and instead focused on designing a kitchen-diner knock through ourselves, the results of which you can read in our full kitchen diner renovation post.


We learned some huge lessons throughout this whole process, but in particular we realised how ‘on our own’ we felt. If you bring in an architect to help you it’s quite likely that they will get carried away with the spend when it’s possible that you haven’t really even decided (or even discussed) what you actually want.

That’s one reason why we devised the How to Renovate a House Online Course which has now helped many renovators to focus on:

  • Identifying their wants and needs

  • Empowering them to make effective decisions

  • Creating design ideas themselves

  • Creating a solid plan for their renovation

  • Saving money, reducing stress and getting amazing results.

We are still so pleased with our decision to not go ahead with the extension but are equally glad that we went through the process of considering doing it. We can only imagine the ever present feeling of ‘what if?’ if we had not investigated the cost of an extension.