Timber Frame Rear Extension: From Foundations To Finished Kitchen

Adding a rear extension is one of the most effective ways to transform a home. You gain extra living space, can reconfigure the layout, and often end up with a much better connection to the garden. In this blog we’ll look at a typical timber-framed rear extension of around 3 metres deep and 5.5–6 metres wide, built onto the back of a house.

• Moving a manhole and sorting drainage

• Building pad foundations and the oversite

• Erecting the timber frame and roof

• Installing steelwork and insulation

• Plastering, screeding and finishing the interior

• Cladding, roughcast and patio outside

• Fitting the new kitchen and tiling

Think of this as a structured “start to finish” guide rather than a strict specification. Local regulations, ground conditions and design details will always vary, but the logic and sequence are very similar.

1. Planning the extension and foundations

This project is a 3 m deep rear extension, running the full width of the house. Because it’s timber-framed rather than full brick and block, the structure is lighter. That allows a slightly lighter foundation design, provided it’s signed off by an engineer and building control.

Instead of a continuous trench foundation, this build uses:

• Concrete pad stones at key load points

• Precast concrete beams/lintels spanning between the existing house foundation and the pads

• Blockwork on top to bring everything up to damp-proof course (DPC) level

The foundation design always comes from structural calculations, but the principle is simple: the loads from the timber frame and roof are carried down onto the existing house footing on one side and the new pad foundations on the other.

2. Moving the manhole and sorting drainage

One of the first jobs is drainage. In this case, an existing manhole and sewer line sit directly under where the extension is going. You can’t build over a manhole, so it has to be relocated.

Key steps:

1. Agree the alteration with the sewer authorityYou typically need permission and sometimes inspections, since the pipe is shared or adopted.

2. Install a new inspection chamber outside the footprint

   • Cut into the main sewer line at the agreed position.

   • Fit a modern inspection chamber and pipework.

   • Make sure inlets and outlets line up and seal properly.

3. Divert or extend the existing pipe runs

   • Temporarily block the sewer upstream so you can work safely.

   • Break out the old clay pipes where the old manhole is.

   • Extend them in a straight run through the new extension area using PVC pipe and flexible couplings.

   • Add new branches for kitchen waste and rainwater gullies as required.

Once the new chamber is installed and everything flows properly, the old brick manhole can be demolished and the pipe below the new extension is just a straight run.

3. Pad foundations, beams and blockwork

With drainage sorted, the foundations can go in.

Concrete pad stones

• Excavate pad locations to the depth and size specified by the engineer.

• Cast concrete pads, well-compacted and levelled.

• Allow them to cure.

Precast concrete beams/lintels

The beams are used as ground beams to span between:

• The existing house foundation, and

• The new pads at the outer corners and midpoints

Some beams also span between the two corner pads, forming a rectangular “ring” at the edge of the extension.

At the internal right-angle corners, both beams bear on the same pad stone, so that pad carries the combined load. The beams are carefully levelled so the top forms a consistent plane.

Blockwork up to DPC

Between and inside the beams, blockwork is laid to form the perimeter wall and internal supports:

• One or two courses of dense blocks are built up to damp-proof course level.

• In tight areas near the neighbour’s wall, brick is used instead of rendered block, because brickwork is easier to keep weatherproof where access is limited.

• A DPC strip is laid on top of the final course, ready to receive the walls.

At this point, building control typically inspects the foundation excavations and the DPC level.

4. Oversite, sub-base and concrete slab

Next comes the oversite – the build-up that will become the internal floor.

Removing old patio and hardcore

• Any old paving, sand and loose hardcore inside the footprint is dug out to the required depth.

• The aim is to remove soft, loose or contaminated material and create room for the new sub-base and slab.

New sub-base

• A well-compacted hardcore layer is added (crushed stone or similar).

• This is compacted in layers with a plate compactor.

• Vent pipes are installed where necessary to maintain underfloor ventilation, especially if the original house has suspended, ventilated floors.

Damp-proof membrane and concrete slab

• A damp-proof membrane (DPM) is laid over the sub-base, lapped up the sides and taped as needed.

• A ready-mixed concrete is wheeled in and poured over the DPM.

• The slab is levelled and roughly floated. It doesn’t need to be perfectly flat at this stage because a screed will be added later.

In this example, the slab is around 125–150 mm thick, with insulation and screed planned above it.

Once the slab has cured, the extension has a solid base and it’s time for the timber frame.

5. Building and raising the timber frame

The new walls are timber-framed rather than masonry. This speeds up the build and keeps weight down.

Assembling the wall frames

• Wall panels are built flat on the slab:

  • Bottom and top plates cut to size

  • Studs, headers and cripple studs installed where doors and windows will go

• The first panel is the side wall along the neighbour’s boundary.

• Once a wall frame is nailed and checked, it’s lifted into place with two or more people.

Fixing the frame to the house

• The side frames are fixed to the existing house wall using bolts and chemical anchors.

• The rear frame is built in place, joining the side frames and forming the new external wall line with openings for French doors and kitchen window.

This “stick-build on the slab, then lift and fix” approach is fast and precise.

6. Roof structure and deck

With the wall frames standing, the next step is the flat or low-pitch roof.

Roof joists and wall plates

• Wall plates are fixed securely to the existing house using chemical anchor bolts.

• Roof joists are hung from these plates using joist hangers.

• The other ends of the joists sit on the front timber wall.

Where rooflights are planned:

• Joists are doubled up and bolted together around the opening to carry extra load and stiffen the roof.

Firrings, ventilation and sheathing

To create a fall on a nominally flat roof:

• Tapered firring pieces are fixed on top of the joists to introduce a gentle slope.

• Counterbattens and cross-battens are added to maintain ventilation gaps where required.

• Structural board (such as 18 mm OSB) is then fixed to form the roof deck.

The roof is finished with a glass-reinforced plastic (fibreglass) system in this example:

• Perimeter fascias and drip trims are fitted.

• All trims and edges are nailed or screwed well.

• GRP trims, matting and resin are pre-cut, then laid out in order.

• The resin and matting are applied in sections, starting from the gutter edge and working back.

• A paddle roller is used to consolidate the fibre, eliminating air bubbles and pinholes.

When cured, this forms a fully waterproof roof shell.

7. External sheathing and weather protection

As soon as the roof is watertight, the walls are sheathed.

• Structural boards (e.g. 11–12 mm) are fixed to the outside of the timber frame. This stiffens the structure and stops racking.

• A breathable external membrane is then wrapped around the sheathing, lapped and taped at joints.

• Windows and doors are fitted into prepared openings, with flashings and sealants to keep water out.

At this stage the extension is essentially weathered-in, and interior work can progress even if the weather turns.

8. Insulation, services and steelwork

The next focus is structure and performance inside the shell.

Roof and wall insulation

The roof build-up in this project is particularly well insulated:

• Two layers of rigid insulation are installed between and below joists, totalling around 175 mm.

• A reflective quilt or foil-faced layer is stapled to the underside, with joints taped. This serves as both insulation boost and vapour control layer.

Walls are insulated between studs with rigid or semi-rigid boards, trimmed to fit neatly.

Cross-battens are fixed under the roof insulation, creating a service cavity for cables and downlights so you don’t have to cut into the insulation layer.

Structural steelwork

To open up the interior and remove internal walls, substantial steel beams are added.

Key steps:

• New padstones are built into walls where beams will bear.

• Temporary props (“acrows”) and lifting gear (such as a genie lift) support the existing structure.

• Old, undersized and rusty steels are cut and removed in manageable sections.

• New, treated steels are brought in, lifted into place and seated on padstones.

• Beams are bolted together where they intersect, forming a rigid frame that carries both the new roof and existing upper floors.

In this case, one main rear beam carries the existing rear wall above, while another parallel beam supports the new roof. A perpendicular beam runs into the existing living room, supporting bathroom walls above once the old wall between spaces is removed.

All of this is done to a structural engineer’s design and signed off by building control.

9. Internal boarding, vapour control and fire protection

Once the structure and insulation are in place, internal linings go on.

• Ceilings are battened and plasterboarded.

• Walls are plasterboarded. In some areas, additional plywood sheathing is fixed behind the plasterboard to provide extra strength for kitchen units or other loadings.

• A vapour control layer is added on walls where required (the foil insulation in the ceiling already acts as a vapour barrier there).

Steel beams that will be boxed in are encased in fire-rated plasterboard (often pink-coloured boards). Exposed steel elements are coated in an intumescent (fire-protective) paint as required by regulations.

10. Plastering and finishing the shell

Now the room starts to feel like a real space.

Bonding and skim coat

• Any deep chases, padstone surrounds and awkward areas are filled and levelled with a base coat plaster.

• Joints are taped to reduce the risk of cracks.

• A two-coat skim of finishing plaster is applied:

  • First coat to cover and roughly level

  • Second coat to refine and polish once the first has firmed up

Tools like long feather edges or speed skims help flatten large ceilings and walls efficiently.

After a couple of days, the plaster is ready for decorating.

Coving and paint

• New lightweight coving is installed with adhesive, matching existing rooms where needed.

• Once plaster is fully dry, mist coats and top coats of paint go on the walls and ceilings.

At this stage, the extension feels almost finished internally, but there’s still work to do on floors and outside.

11. Insulation, membranes and screed to the floor

The floor build-up is completed once the shell is dry and weather-tight.

• Rigid floor insulation boards are laid over the slab.

• Services such as kitchen waste pipes and heating pipes are run where they need to go.

• Perimeter upstand insulation is added around the edges.

• A second damp-proof membrane is laid over the insulation. This separates the insulation from the screed and protects against moisture.

A semi-dry sand/cement screed is then mixed and laid by hand:

• Screed is levelled with straightedges and compacted.

• This creates the final surface for floor finishes like tiles, timber or vinyl.

Once cured, the floor is solid and ready for its final finishes.

12. External finishes: patio, cladding and roughcast

Outside, the new extension needs to blend with the existing house and garden.

Rebuilding the patio

• The old patio area was under the extension, so a new one is created just beyond the new rear wall.

• Hardcore and sharp sand are laid and compacted.

• Existing patio slabs are relaid in the original pattern but further out, giving a 3 m deep terrace.

• The slabs are laid dry and later compacted, then kiln-dried sand is brushed into the joints.

Fire-rated boards and roughcast

To meet fire and weather requirements:

• Cement-based fire-resistant boards are fixed over battens around the exterior.

• A base coat render or dedicated roughcast backing coat is applied.

• While still wet, pebbles or stone chippings are dashed into the surface to create a roughcast finish.

• Once cured, the render is painted to match the existing house.

Gutters, downpipes and external lights are then refitted or added as needed.

13. Kitchen installation and tiling

The final phase is what people notice most: the kitchen and finishes.

• Kitchen carcasses are installed and levelled.

• Temporary worktops and sink may go in while waiting for stone or composite tops.

• Once permanent worktops arrive, they’re fitted along with the sink and tap.

• Appliances like range cookers, dishwashers and washing machines are connected and tested.

For the splashback:

• Subway/metro tiles are a popular choice. They’re laid in a brick bond up to a chosen height, around windows and under wall units.

• External corners are finished with trim.

• Gaps that are too narrow for a full tile or grout are built up with a stable backing and later covered with trim.

Once the adhesive has cured, the tiles are grouted. Silicone sealant is applied at worktop junctions, around sinks, and at internal corners for flexibility.

The last bits of carpentry, trims and paint are finished, and the extension is effectively complete.

Final thoughts

A timber-framed rear extension like this involves a long chain of coordinated steps:

• Groundworks and drainage

• Foundations and oversite

• Timber frame, roof and waterproofing

• Insulation, steelwork and structural details

• Plasterboard, plaster and screed

• External roughcast and patio

• Kitchen fit-out and tiling

When each stage is done methodically and inspected where required, the result is a warm, bright, open-plan space that feels like it’s always been part of the house – but performs much better in terms of layout, insulation and everyday living.