How to Convert a Loft

Want to add space and value to your home? Follow Paul Hymers' step-by-step guide to building a loft conversion
by Paul Hymers
29th March 2021

Before you begin on your loft conversion plans, it’s worth finding out exactly what it will involve and how it will affect the design of your home.

This step-by-step attic conversion guide reveals the 10 key things you need to consider when weighing up whether a loft conversion is the right way to add more space to your property.

Ensure your loft conversion design ideas take the following things into account:

1. Can your home take the weight of a loft conversion?

Clearly building a loft conversion will add weight to your house and, although it may only be a modest increase, you’ll need to make sure that the structure of the building can take it.

To do this, you or your builder will need to expose the foundations and check them, together with any beams or lintels that will be asked to carry more weight.

Your Building Control officer will also want to check all these elements, so dig a small hole to expose the foundations first.

If it turns out that your house needs underpinning to support the extra weight, it could double your budget before you start. So this is a key consideration when planning how to convert your loft.

2. Is there enough head height for conversion?

Get your designer to illustrate clearly how much headroom you’ll have in your loft once it’s converted – people are often disappointed by how much space they have to actually stand up in and on plans this isn’t always clear.

Don’t forget you’ll have to accommodate a staircase leading up into the loft. To make the best use of space the new staircase should rise above the old one and not from within an existing bedroom. There’s not much point in converting the loft space if it means losing an entire room on the first floor.

Without the roof space for water tanks and plumbing, the heating and hot water system may have to be replaced with a sealed system. It’s better to have an unvented hot water cylinder than a combination boiler, but it will take up a cupboard-size room and you’ll need to find somewhere to put it.

EXPERT VIEW: Calling in quality loft conversion quotes

One of the key steps in the search for the right builder is understanding and comparing quotes. Deepak Sing Udassi from City Lofts London reveals why obtaining quality quotes is all about the planning:

Start by writing a design brief. Think about why you are undertaking this project and detail the must-haves, should-haves and could-haves. Then invest in quality drawings. For simple, non-structural home improvements you could probably go straight to the builder; for anything else hire an architect to produce detailed plans.

The next step is to shortlist prospective builders. Seek recommendations from friends and family, and find accredited professionals through the Federation of Master Builders’ Find a Builder service. Invite a minimum of three builders to quote. Give the same detailed plans to each builder, and ask them the same questions, so you can easily compare quotes.

Check each quote you receive contains plans referencing everything in the scope (as well as things that are considered out of scope) and a full fixed price (stating whether it includes VAT). It should also detail any assumptions or special considerations the builder has made about the project.

Before you accept, carry out your own due diligence. List any concerns and have your builder reply in writing. Request references, visit past or current projects, and check their credentials (such as FMB registration) and insurance documentation.

The quotes you receive will only be as good as the pre-work you put in, the thoroughness of the plans you provide and the suitability of your builder shortlist.

Deepak Singh Udassi is MD of City Lofts London, a Master Builder company that specialises in high-quality loft conversions. He also runs RIBA architectural practice Architecture 100, specialising in house extensions. This 360-degree experience gives Deepak a valuable, end-to-end overview of the house extension process.

3. Building Regulations & Party Walls for loft conversions

Loft conversions always need approval under Building Regulations (irrespective of whether they need planning permission) so it pays to adopt the full plans application approach and have a detailed scheme approved before you find a builder.

Having an approved design will take much of the risk out of the work and also mean the builder has a chance to give you a fixed quotation, rather than a vague estimate.

If your house is semi-detached or terraced don’t forget to notify your neighbour of your proposals, which will usually fall under the Party Wall Act 1996.

Your Building Control officer will inspect the work at various stages and on a final inspection should issue you with a completion certificate. Don’t settle any final accounts with contractors until you’ve received the certificate.

4. Altering the roof structure & floor joists

Most roofs are constructed with internal support struts in the loft, propping up the rafters and purlins (horizontal roof beams) in traditional cut and pitched roofs, and making up the web of braces in modern trussed rafter roofs.

All these have to be removed to make way for the new room and replaced with new supports that don’t encroach on the space available in the loft void.

How much will a loft conversion cost?

This cost guide is provided by the Build It Estimating Service. It is based on a straightforward dormer loft conversion with a useable internal floor area of 20m2.

Loft elementCost
Material cost£17,160
Labour cost£17,550
Plant cost£1,950
Scaffold & subcontract cost£2,340
Total build cost (excl. VAT) £39,000
Loft conversion build cost per m2£1,950
Total cost (incl. VAT) £46,800

There are many ways of altering roof structures for loft conversions, but they all have one common element –the ceiling joists will almost certainly be inadequate as floor joists. This means that new floor joists are fitted alongside them, slightly raised above the ceiling plasterboard to avoid contact with it.

These joists (often 200mm or 225mm in depth) will rise above the tops of the current ceiling joists to form the floor structure. Depending on their span they will bear either directly on to the existing wall plates of external and internal load-bearing walls, or on to newly installed beams.

New Bedroom and Ensuite in Dormer Loft Conversion

Dormer loft conversion by Carai

When Gary and Linda wanted to provide enough room for an elderly relative to move in, they were keen to maximise the potential for extra habitable space in their home’s loft.

They wanted to create a space that was separated from the house, which could provide a peaceful haven. The large use of glazing ensures that this cosy room remains airy and bright.

Read the full story

In smaller lofts, it is often the case that the floor joists themselves will be used to support the sloping rafters. This is possible by constructing a dwarf timber stud wall 1m to 1.5m high, known as an ashlering, between the two. With the supporting ashlering in place, the internal struts and braces can now safely be removed.

5. Loft conversion stairs

Stairs are invariably tricky to design on loft conversion projects, as space for them is tight. Narrow winding flights are acceptable, but may prove impractical, because it’s difficult to get furniture up them.

Purpose-built staircases are around 10 times the cost of standard (off-the-shelf) designs, so bear this in mind when you’re planning your loft conversion.

If you do need a purpose-built loft conversion staircase, it pays to have the design approved by your Building Control officer before you actually commission them. Ask your joiner or builder to send Building Control a copy of the design.

As part of the fire safety upgrade for your loft (see below) your stairways should lead to a hall and an external door. If you have an open-plan arrangement where the stairs rise from a room, it is likely you’ll have to alter it, fitting a new partition wall or choice of escape routes.

6. Fitting windows & gaining natural light

You don’t need to make a lot of structural alterations to accommodate rooflight or skylight windows in your new loft conversion, which makes them relatively easy to fit. Typically the rafters on either side of the rooflight are doubled-up and trimmed across the top of the opening.

A popular alternative is to fit dormer windows, which are structures in themselves, as they have walls and a roof as well as the window itself.

1930s House Transformed by Loft Conversion

Bright lounge in renovated 1930s home

Adrian and Maria Pearcey converted their loft to make space for an extra bedroom. For the new master bedroom in the attic, the couple wanted to maintain a sense of being up in the eaves of the dwelling. “We decided against the full-width dormer because we didn’t need a huge space at the expense of losing that wonderful lofty feeling,” says Maria.

The couple did, however, install a cottage dormer to give enough headroom for a new ensuite bathroom. The zone feels bright and spacious with four large Velux windows allowing copious amounts of light to stream into the bedroom.

Read the full story

At the rear of many homes dormer windows can fall into the permitted development quota and so may not require planning permission. At the front of the house, however, they will require planning permission, which is why you often see rooflights or skylights instead.

Dormer windows may be essential to maximise the headroom in the loft and provide useable space, but will need to be supported at the apex point (ridge). A ridge beam is installed beneath the apex before the dormer roof joists can themselves be fixed in place and the roof weathered.

It is at this stage, when the dormer windows are being constructed, that your loft conversion will be exposed to the elements, so you’ll need good temporary sheeting to protect against the weather.

7. Upgrading loft fire safety

Loft conversions on bungalows have little effect on the fire safety of your home, beyond making sure that the new windows are large enough to escape out of. But in a house where two storeys become three, there are implications.

The new floor will need at least 30 minutes of fire-protection, which could mean re-plastering the ceilings below it and the loft room will have to be separated by a fire door, either at the top or bottom of the new stairs. You’ll also need one escape-sized window per room – some skylight windows are made specifically with this in mind.


When Sue Cambie renovated her home in Brighton, she wanted to create a welcoming abode with contemporary, yet quirky interiors. She completed her dream project for £52,670 – including a loft conversion with master bedroom and bathroom, which added 27m2 to the house’s total floor space.

Read the full story

Door self-closing devices are no longer required in homes. They’ve proven to be a risk to children’s safety because they can trap tiny fingers. Instead, existing doors on the stairway (ground and first floor) should be replaced with fire-resistant doors or upgraded – and this should be indicated on your loft conversion drawings for Building Control.

As part of the electrical installation, mains-powered smoke alarms should be installed on each floor of your home and these should be interlinked so that they all sound when one is activated. Most have a re-chargeable battery as a back-up that allows the supply to be extended from a lighting circuit if need be.

8. Loft conversion insulation

With energy efficiency standards being increased, loft conversion insulation is more difficult to install than it once was. If you are replacing the roof tiles at the same time, you can insulate between the covering and the rafters, which will also achieve good airtightness.

If you’re not replacing the roof, the sloping ceiling will need insulation cut and fitted between the rafters, as well as on the underside of the rafters. As the plasterboard will have to be fixed to the rafters through the bottom layer of insulation, you will want this insulation to be as thin as possible.

You should use some high performance insulation (typically a foam board) for all of these areas. The ashlaring walls and dormer window structures will also need insulating with similar products before they are plasterboarded.

9. Sound insulation for your new loft conversion

The new floor also needs soundproofing, and this is easily achieved by laying a mineral fibre quilt between the joists. Use the heavier, denser sound insulation quilt and not the lighter thermal insulation material, which is of no help here. The same goes for any internal stud partitions between bedrooms or bathrooms.

You should also consider insulating any party walls, both against heat loss and noise. A lining framework of timber stud will allow you to achieve both and you can cover it with sound-rated plasterboard.

10. Loft storage ideas

When you convert your loft you are, of course, going to lose storage space. Make the most of what you have by using the eaves behind the ashlaring – fit access hatches and have roll-out storage bins made to fit. And if you insulate down the rafter line to the eaves you’ll create a warm store for your belongings.

Built-in wardrobes are also a great feature in loft bedrooms, where standard units won’t fit – and are among the most innovative loft storage conversion ideas.

Main image: Velux

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