Trust Mark

read more testimonials...

loft extension


loft extension


loft extension


loft extension


loft extension


loft extension


loft extension


loft extension

How much will a loft conversion cost?

Each loft conversion is very different due to the differences in each house and each loft. However, we can provide a rough guide to give you an idea of what will increase and decrease the cost of a loft conversion.

Velux Loft Conversion Costs

Here we will try to give you an idea of how much you will be charged for a basic loft conversion using Velux windows for light and ventilation.

What you will need to know:

Velux Loft Conversion1. An approximate volume of the entire room /rooms within the loft
The costs here are between £350 per CM and £500 per cubic metre
2. How many and what design of Velux windows you would like
We have added a link to as there are many different designs available, the costs are itemised on this site.  Installation costs ae covered within your base quote (with the exception of Cabrio and balcony Velux systems)
3. Your electrical requirements
Please add £900.00 for a standard bedroom installation of 2 X double power points and 1 X light and switch, along with the necessary 3 X interlinked smoke detectors to satisfy local authority.  Please add £350.00 for bathroom electrics.   Please add £55.00 per unit for requirements over this standard.
4. Your plumbing requirements
Please add £400 for a standard radiator added to an existing combination boiler system. 

Please note these costs are set up as a guide starting point and are not meant to give a guaranteed quotation, a free site survey will be needed to assure costs.
Example 1

Creating a loft room of 40 cubic metres Type “D” in a standard detached house:

40 cubic metres Type “D” (40 cubic metres x £400)


3 x GGL SO6 Velux with flashing kit EDZ    


Electrical requirements   


1 x Radiator   


2 x extra double sockets in bedroom for computer   


Add costs for scaffolding and skips (approximately)


Sub total:




N.B. Please add VAT at current rate
N.B. Please add 15% to your loft conversion cost for inner London areas.

So you could add a spare bedroom / home office / kids playroom from as little as £15,000.00 +VAT inclusive of Architect’s plans.

Dorma Loft Conversion Costs

sbsIn the past dormers have been referred to as “Dormer windows” this was typically a  traditional window sited on a roof structure, see example below:

However over the years, and due to customer demand, these dormers have often grown from small protrusions within the roof structure to more elaborate sizeable structures that can really expand the internal space of a loft or attic. 

There are many factors to be considered when deciding on how big or how small your dormer should be, and most importantly the exterior design, A rep or Architect from Luxus will help you with this should you require, however for now we are trying to give you a basic cost analysis for these extensions.

sbsFirstly you need to know if the dormer will have a flat roof or a sloping roof as the example above. Secondly you will need to know what materials will make up the exterior surface of the dormer. Lastly you will need to know an approximate volume the dormer extension will take up (this volume is the amount in cubic metres the dormer has grown out of the original roof structure, not the entire volume of the loft room).

The following is a basic mathematical equation that should lead to a base point for costs for your loft conversion, over and above costs already estimated for the conversion earlier.

A - Cost per cubic metre (CM) for dormer extensions (general)


B - Cost per CM for dormer extensions built in brick


C - Cost per CM for dormer extensions built in timber with tile cladding


D - Additional Charges for sloping roofs costs per CM


E - Add the cost of the glazed window within the dormer:
Approximately £250.00 per square metre typical window
1.2M wide  X 1.0M High = 1.2 square metres (£250 X 1.2)


Example 1

You have been given an online or posted quote for a loft conversion without dormers that initial quote is £14,750.00 +VAT.
You would like to add a small dormer to possibly house a bathroom to the loft conversion. The dormer will be similar to the example shown, approximately 2.0M wide at the front elevation, 2.2M high at the front elevation and 3.0 long at the ridge line.  This would produce a volume of approximately 6.6 cubic metres.  You would like this dormer with a sloping roof and built from brick.
A - 6.6 CM X  £310.00 = £2,046.00
B - 6.6 CM X  £150.00 = £990.00
D - 6.6 CM X  £165.00 = £1,089.00
E - pvc window = £300.00
Total = £4,425.00


Typical Loft Conversion Costs

The cost of a loft conversion does of course depend on how large your loft is, how many rooms you want and the level of finish and fittings you specify. But, as a guide, most loft conversions cost between £17,000 and £50,000. With Smiley Building Services, you will receive:

  • A fully inclusive fixed price estimate
  • All works under a fixed price contract with fixed timescales
  • A full Scope Of Works agreed by all parties

Our prices are inclusive of design, drawings, building specifications and all building works to a fully decorated finish, if appropriate.