Does a solid roof make it an extension?

Most of us picture a conservatory as a light, mostly glazed structure that sits outside the main house. The roof is a large part of what makes it that. Glass or polycarbonate lets daylight in and keeps the weight low. Once a solid roof goes on, with rafters, insulation, a covering and a plastered or lined ceiling, the building stops looking like a conservatory and starts to work like a room.

Homebuilding and Renovating (updated July 2023) treats swapping a conservatory's roof structure for a solid one as material alteration, and that puts the job under the Building Regulations. Its advice is to submit a building control application and have an inspector check the work. That is the clearest published line we have on the status question: the act of fitting the solid roof is what brings the Regulations in. We do not need to settle whether the building has become an extension in law to know that the work is notifiable.

The Planning Portal expects a new conservatory roof to be safe in structure, efficient with energy and properly ventilated. Those three tests make a handy checklist for what a building control officer or approver will examine on a solid roof.

Structure

A solid roof can weigh far more than the roof it replaces. The existing frame, the dwarf walls and the foundations are checked to see whether they can carry it, and reinforcement may be needed. Approved Document A treats a covering that adds about 15% to the roof load as a significant change, so a heavier roof is a prompt for a check.

Energy

Part L of the Regulations sets limits for new and replacement roof elements in existing dwellings, with a roof limit of 0.15 W/m²K in Table 4.2 of the 2021 edition. Whether that figure applies to your build is for building control to confirm. Insulation therefore moves from a nice extra to something that is inspected. Our guide to Building Regulations for a solid conservatory roof covers each requirement.

The practical way to think about status is by what the building does for the house. A conservatory that is separated from the house by doors, and has its own heating arrangements, is treated differently from a room that opens straight into the house with no separation and shares its heating. A solid roof makes the second arrangement much easier to live with, which is why so many owners combine the two.

Heating is a good example of the grey area. A conservatory with its own radiator or underfloor loop on a separate control is one thing. One that shares the house heating and has no door between is quite another, and once a solid roof and insulation are in, the second arrangement is far more tempting. Tell building control what you intend at the outset, because a change of plan after approval can mean going back to them.

We cannot tell you, from a phone call, which category the council or an approver will place yours in. They look at how the conservatory is separated, what it is built from and how it is used. Our part is to photograph the conservatory and record the existing frame, walls, doors and heating, and give you a clear written description to take to building control. Ask them in writing how they classify the finished building, and keep the reply with your papers. It will matter when you come to sell. Buyers' solicitors tend to ask for the completion certificate, and without it a sale can stall while someone chases the council for retrospective paperwork.

Status for Building Regulations and status for planning are separate questions. The Planning Portal adds that planning permission could come into play when the roof changes substantially in structure or appearance. That is covered in our answer on planning for a solid conservatory roof.

On the Building Regulations side, you can go to the Gloucestershire Building Control Partnership (Stroud's shared service with Gloucester City) or choose a registered building control approver. Whenever the job is notifiable, we explain the route, send the notification and fit the inspection points into the programme. When the work is finished you should receive a completion certificate. Keep it safely, because it is the evidence that the roof was properly approved.

If what you want is a warmer, more usable room, a solid roof is one route, and a very good one. If what you want is just a roof that stops leaking, a new polycarbonate roof is simpler and quicker, and it raises fewer of these questions. If one sheet has failed but the roof is otherwise sound, our answer on replacing a polycarbonate sheet may cover it.

Self-cleaning glass is another option for keeping the daylight. Our answer on self-cleaning conservatory glass sets out what it does and does not do. The conservatory roof replacement page shows how we survey and quote, and a solid roof comes with our 10-year workmanship guarantee in writing.

Whichever way you lean, settle the status question early. It shapes the drawings, the insulation, the fixings and the order in which things happen, and it is much cheaper to answer on paper than once the old roof is already off.