{"id":25349,"date":"2026-10-13T06:39:38","date_gmt":"2026-10-13T04:39:38","guid":{"rendered":"https:\/\/boisetlendemains.fr\/extension-ossature-bois-ou-parpaing\/"},"modified":"2026-10-13T12:41:26","modified_gmt":"2026-10-13T10:41:26","slug":"timber-frame-or-concrete-block-extension","status":"publish","type":"post","link":"https:\/\/boisetlendemains.fr\/en\/timber-frame-or-concrete-block-extension\/","title":{"rendered":"Timber frame or concrete block extension: how to choose when extending your house"},"content":{"rendered":"

Extending rather than moving is often the most sensible decision when a family grows or a home office becomes a necessity. Then comes the question that shapes the rest of the project: should the extension be built in concrete block, like the rest of the house, or should you opt for a timber frame extension? Both techniques are proven, insurable and built to last. They do not, however, answer the same constraints. Here is how to weigh them up honestly, with a particular eye on Toulouse houses.<\/p>\n

Two techniques, two ways of building<\/h2>\n

Concrete block belongs to wet construction<\/strong>: foundations, block walls laid in mortar, ring beams, then insulated lining on the inside and render on the outside. Each stage needs time to set or dry before the next one can start.<\/p>\n

A timber frame extension<\/strong> belongs to dry construction<\/strong>. The walls are made of evenly spaced vertical studs, braced by a sheathing board. The insulation sits between the studs, within the thickness of the wall itself. Outside, a breather membrane protects the whole assembly, then a ventilated cavity separates the wall from the cladding: timber boards or a render board. Inside, a continuous vapour control layer manages moisture before the internal finish. In France, the work is governed by NF DTU 31.2, the reference standard for timber frame buildings.<\/p>\n

This difference in approach explains most of what follows: a timber wall is a build-up of layers, each with a precise job, whereas a block wall is first and foremost a load-bearing mass that is then insulated.<\/p>\n

Timber frame or concrete block extension: a point-by-point comparison<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n
Criterion<\/th>\nTimber frame<\/th>\nConcrete block<\/th>\n<\/tr>\n<\/thead>\n
Build time<\/strong><\/td>\nShort: no drying time, walls sometimes prepared in the workshop<\/td>\nLonger: setting and drying time between stages<\/td>\n<\/tr>\n
Weight<\/strong><\/td>\nLight: foundations often simpler<\/td>\nHeavy: foundations sized accordingly<\/td>\n<\/tr>\n
Wall thickness<\/strong><\/td>\nThinner for the same thermal performance, as the insulation sits within the wall<\/td>\nThicker: the insulated lining is added to the block<\/td>\n<\/tr>\n
Winter insulation<\/strong><\/td>\nVery good, with fewer thermal bridges<\/td>\nGood, provided junctions with slabs and cross walls are dealt with<\/td>\n<\/tr>\n
Summer comfort<\/strong><\/td>\nDepends heavily on the insulation chosen<\/td>\nThermal mass, if it sits on the inside<\/td>\n<\/tr>\n
Disruption<\/strong><\/td>\nLittle rubble, little noise, a clean site<\/td>\nA heavier site, and a longer one next to an occupied house<\/td>\n<\/tr>\n
Appearance<\/strong><\/td>\nTimber cladding, or render on a suitable board<\/td>\nRender, in keeping with a masonry house<\/td>\n<\/tr>\n
Points to watch<\/strong><\/td>\nWater management, continuity of membranes<\/td>\nThermal bridges, drying time before finishes<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Build time matters more than it seems<\/h3>\n

An extension is almost always built against an occupied house. Every week on site means an open wall, an unusable terrace, noise and dust. Built dry, a timber frame is quickly weathertight, and the following trades can move in without waiting for walls to dry out. The gain is not just counted in days: it is counted in weeks of everyday life with less disruption.<\/p>\n

Thinner walls, more floor area<\/h3>\n

In a block wall, the insulation is added to the block. In a timber frame, it fills the space between the studs. For the same thermal performance, the timber wall is therefore thinner. On an extension, where the footprint is often limited by the plot or by planning rules, the centimetres saved on each wall become usable floor area.<\/p>\n

Summer comfort: not to be overlooked in the south-west<\/h3>\n

This is the strongest argument in favour of masonry, and it would be dishonest to skip over it. A heavy wall absorbs heat and releases it later: this is thermal mass. A timber frame insulated with a lightweight material has little of it, and a south-facing room can overheat in summer.<\/p>\n

There is an answer, but it has to be decided at the design stage. Dense insulation such as wood fibre markedly slows the passage of heat through the wall. A well-sized roof overhang, external solar shading and the option of night-time ventilation do the rest. A timber extension designed for the Toulouse summer is comfortable; a timber extension designed like an opaque conservatory is not.<\/p>\n

Cost: less of a gap than you might think<\/h3>\n

Two received ideas circulate: “timber costs more” and “timber is cheaper”. Serious comparative studies, such as the one published by the French state services in Nouvelle-Aquitaine, find small differences that swing one way or the other depending on the level of performance targeted. What really drives the price of an extension lies elsewhere: ground conditions and foundations, the junction with the existing house, the level of finish, the windows and doors, and the services to be extended. Comparing two quotes only makes sense when the scope and performance are identical.<\/p>\n

The Toulouse case: adding timber to a brick house<\/h2>\n

Much of Toulouse’s housing stock is built in brick, sometimes combined with Garonne river pebbles: suburban “toulousaines”, single-storey terraced houses, town houses. These old walls have one particular feature: they exchange moisture with the surrounding air. Sealing them in, or placing them in direct contact with a material that traps water, causes problems.<\/p>\n

The junction with the existing house<\/h3>\n

This is the trickiest part of any extension, whatever the technique. Old and new do not move in the same way: the existing house has finished settling, while the extension has barely begun. The two structures are therefore separated by a joint designed to absorb these movements without letting water in. A timber frame lends itself well to this: it is light, and its connections to the old wall can be made without damaging it.<\/p>\n

Clay soil<\/h3>\n

Toulouse is covered by a risk prevention plan for clay shrinkage and swelling: in dry periods the ground shrinks, in wet periods it swells, and shallow foundations bear the movement. The lightness of a timber frame does not remove the need for suitable foundations, but it reduces the loads to carry and the stresses at the junction with the house. The exposure of your plot can be checked on the French G\u00e9orisques<\/a> website, and it is worth knowing before the foundations are designed.<\/p>\n

Appearance: cladding or render<\/h3>\n

A timber extension does not have to look like a chalet. Set against a brick fa\u00e7ade, it can embrace the contrast with vertical cladding in a durable species, which will silver and mellow over time. It can also receive a lime render<\/a> on a wood fibre board designed to be rendered, and blend into the street. In a conservation area, it is often the heritage architect (Architecte des B\u00e2timents de France) who steers this choice.<\/p>\n

\"Timber
The breather membrane, then cladding fixed on battens: the air gap between them lets the wall dry out after every shower.<\/figcaption><\/figure>\n

Permissions<\/h3>\n

The technique makes no difference to the paperwork: it depends on the floor area created and on where the plot is. In the usual case, a prior declaration is enough between 5 and 20 m\u00b2, a threshold raised to 40 m\u00b2 for extending a house in an urban zone covered by a local plan (PLU). Above that, full planning permission is needed, and using an architect becomes compulsory if the total floor area exceeds 150 m\u00b2 after the works. In protected areas, the rules are stricter. The details are explained in our article on planning permission in Toulouse<\/a>, and your town hall will confirm what applies at your address.<\/p>\n

Adding a storey: where timber comes into its own<\/h2>\n

When the plot leaves no room to extend outwards, the only way is up. On a single-storey house, the question is no longer really “timber or block”: weight decides. A masonry upper storey puts a load on the existing walls and foundations that many cannot carry without heavy strengthening. A timber frame, being much lighter, makes the project feasible in many cases where it otherwise would not be.<\/p>\n

Feasible does not mean guaranteed: whether the walls and foundations can carry an extra storey has to be checked before any drawing, and it is a structural engineer who confirms it. The existing roof structure is usually taken down, which also makes it the opportunity to rethink the roof entirely.<\/p>\n

What makes a good timber frame extension<\/h2>\n

Timber’s longevity as a structural material needs no proving: roof structures and half-timbered houses have stood for centuries. The durability of a timber frame does not come from the wood itself but from how it is protected from water. A few rules make all the difference:<\/p>\n