What a roof window costs to fit

The unit is the line people look at first, and three choices move it. The first is size. Roof windows are made in a range of standard sizes, and the maker's code gives the dimensions: in a code such as MK04, the letters give the width and the digits the height. A wider window takes more glass and a longer frame, and it may sit across a rafter that has to be trimmed.

Opening style is the next choice. A fixed window has no sash to open and costs less than an opening one of the same size. It suits a stairwell or a high position where nobody would reach a handle. A centre-pivot window turns on a horizontal axis near its middle, and a top-hung window swings out from a hinge along the top edge. Each opens differently and fits a different position in the slope, which our page on roof windows compares in detail. Electric opening and rain sensors add a motor and wiring to the unit.

The third choice is the glass. A new or replacement roof window takes the window limit of 1.4 W/m²K, or Window Energy Rating band B at minimum. Laminated inner panes, tinted or solar control coatings and extra sound reduction each lift the specification, and over a south-facing slope in a bedroom the glass choice can matter as much as the size.

A roof window leaves the covering with a hole to fill, and the flashing is what closes the gap between frame and tiles. It is made for a specific covering, so peg tile, slate and concrete interlocking tile each need their own kit. Plain tiles are small and overlap in many courses, which means the flashing and the covering must be dressed together with care around the frame. Slate is thin and sits flat, and it takes a different set of pieces. Interlocking concrete tiles have deep profiles, and the flashing has to follow the shape of the tile.

The labour differs too. Lifting and relaying hand-made tiles around an opening takes longer than lifting a few large ones, and old slate may need replacing where it breaks under the hand. If the covering is brittle, we will tell you at the survey and write in any slates or tiles to be renewed. Our guide on roof window flashings for Kent roofs explains each type.

Fitting two windows side by side, or one above the other, uses a combined flashing kit that links them. That kit is priced as its own line, and it means the covering between the windows is rebuilt in one go.

On a bungalow, a ladder and a roof ladder may be all that is needed. On a two-storey house with a loft slope at the back, the same window may need a scaffold tower or a full scaffold. Where a terraced house has no side passage, every tool and length of timber goes through the hall. These access costs are priced into the quote.

The opening comes next. A window narrow enough to fit between two rafters needs very little structural work. A wider unit cuts a rafter, and the cut rafter is supported with trimmers so the load goes round the opening. Where the loft has a purlin or a partition in the way, the plan may change. We check the rafter spacing and direction from the loft on the survey, so the window is sized to the roof and not the other way around.

The insulation in the loft slope also has to be kept continuous around the frame. A gap in the insulation next to the glass is a cold spot that gathers condensation on the inside of the lining. Our guide to the minimum pitch for a roof window covers the other check made on the slope, whether the angle suits the window at all.

Inside, a roof window sits at the end of a short shaft cut through the slope. Its sides are lined with insulated board and finished flush with the ceiling, so the window reads as part of the room. In a loft bedroom or a study under the eaves, that lining is usually a small job. In a landing under a steep rafter with a plastered slope, it means matching the existing surface. Splaying the sides gently toward the room sends more daylight across the ceiling.

The room needs protecting while the work goes on. Sheets over floors and furniture, dust screens across the doorway and a clear path for tools all take time to set out and clear away. A finished loft bedroom with new carpet takes more care than an empty loft.

Plastering and paint are separate trades. Our quote states where our work stops, whether that is at plasterboard, at a skim coat or at a paint-ready surface. Decorating after that is for you or your decorator. If an electrician needs to move a ceiling light or fit wiring for an electric window, that is written in as its own line.

A new opening in a roof is building work, and the Building Regulations apply to its structure, its energy performance and, in some rooms, fire safety. We handle the Building Control notification where the work is notifiable, and that fee appears on its own line. Our skylight Building Regulations guide sets out what is checked.

For a loft room the escape route matters. On a two-storey house a loft conversion creates a storey above 4.5 m, so it normally needs a protected escape route down the stairs. Building Regulations do not treat a roof window in that loft as the way out. In a bungalow or chalet bungalow, a roof window can serve as the escape window if the opening is at least 0.33 m² and at least 450 mm wide and 450 mm high, with the bottom of the opening no more than 1,100 mm above the floor and the window staying open without being held. That affects the size and the hinge type, so it shapes the quote.

Planning is the other check. Roof windows on a house are usually permitted development if they project no more than 150 mm beyond the plane of the original roof slope, including the frame, and sit no higher than the highest part of the original roof. A window on a side-elevation slope must be obscure-glazed and non-opening unless its opening parts are more than 1.7 m above the floor of the room. Flats, maisonettes and houses created under change-of-use permitted development have no such rights, and an Article 4 direction or a condition on the original permission can remove them. The answer on whether you need permission for a roof window goes through the cases, and we check yours at the survey.

Lay your quotes next to one another and mark what each one includes. A good quote reads like a description of the work. It names the window by size code and glass, the flashing for your covering, the access method, the structural work, the lining and the paperwork.

Replacing an old roof window follows the same lines. The frame is lifted out, the opening is checked and the new unit is flashed into the covering. A like-for-like swap usually involves less framing than a new opening, and the old flashing has to be taken off carefully. The steps are laid out in swapping an old VELUX window.

A fixed written quote comes with a 10-year workmanship guarantee in writing. To start, send a photo of the slope and the room to WhatsApp 07880 284133, or book a free survey.