RENOVATIONs Calculator

Reonovations Calculator for a 3M x 5M Ground Floor Rear Extension 

Disclaimer: All the information on this page including the calculator below does not constitute legal or financial advice, but are “Considerations”. The amounts below are estimates and is aimed at showing you the significant costs other than the Builders cost when considering a ground floor extension in London, UK.

  • The amounts below are based on a 3M x 5M ground floor extension, with no side extension and is under permitted development and no planning application needed. However,  Building Regulations are required to ensure it is built to standard.
  • These are based on London, UK Prices. Prices outside London may vary and are likely to be up to 20% – %30 lower. 

Choose the approx size of your extension in sqm (L X B e.g 3m x 5m = 15sqm) – This includes 1st Fix: Demolition, Foundation, Walls, Roof, electricals, Insulation, basic flooring (timber joists and floor boards). 2nd Fix: Plaster Board, Internal Roofing, Skimming, pipe and waste work for a kitchen, coving, skirting boards, make the internal ‘good’
The cost for the Architect to generate Architectural Drawings the Builder will use. (Note you do not need Planning permission as there is no side extension and the property is within the 3M rear extension restriction for Permitted Development.
You need building regulations whether you require planning permission or not for a ground floor extension. You could use the council or you could source a private company



Internal Considerations

For15msq give an allowance for £2000: Internal Finishing – Paint, Coving, Skirting Boards, Wall Paper, lights, sockets. This does not include a dinning table set, Television, special Pendant lighting e.t.c
Assuming no critical issues with the waste this is likely to cost around £1800 for the toilet including materials and labour + ground work preparation (waste pipe connection and make good) +£1500
Aluminium External Bifold Doors, 3m x 2m, + Survey and Installation
A Sofa in an open plan extension
Outdoor Table and Chair
2m x 1m Skylight above the new extension
External Patio, Stone based (not porcelain), + installation, ground work, materials + labour
Natural Grass Installation: Ground work, + underlay soil, + carpet grass, + labour, assumes no critical issues with soil
Artificial Grass Installation: Ground work, + underlay soil, + carpet grass, + labour, assumes no critical issues with the groundl

As you can see the builders cost is ‘part’ of the overall cost. The builder may not give you the Overall Cost to Consider. They quote based on their own work.

The significant addition is your kitchen and internal fittings. Some people may ask their builder to fit the kitchen to reduce cost, but some kitchen manufacturers may not insure part or all of your kitchen if it is not fitted by their fitting service. In addition, it is always best to get a specialist kitchen fitter than a builder.