Hi all,
I’m looking for some guidance and shared experience as we move into the early stages of a rear extension project.
We’re proposing a single-storey rear extension, approximately 10m wide x 4m deep (c.40m²). There is a possibility it could become two storeys, but at this stage my wife and I are still undecided. Our intention is to start making progress this year, and from April we’d like to begin with the groundworks so we can at least get something tangible underway.
We are strongly considering a timber frame build, ideally on an insulated slab with underfloor heating within the main slab. At present we’re undecided on the roof design:
Pitched roof (possibly with eaves), or
Flat roof incorporating 1–2 roof lanterns
The rear elevation is likely to have two or possibly three sets of French doors, although we may also consider sliding doors.
We fully expect to engage a structural engineer for calculations, but before getting too far down that road I’m keen to understand best practice from those who have done something similar—particularly from a low-energy / green building perspective.
Some of the specific areas I’d appreciate advice on:
Insulated slab vs traditional strip foundations
Given the footprint (~40m²), would an insulated slab typically be preferable, or are strip foundations still a sensible option in this scenario?
Slab depth, insulation type and thickness
Any guidance on typical build-ups, edge detailing, and thermal performance targets would be very helpful.
Timber frame wall build-ups
From reading previous posts on this forum, a timber frame with OSB sheathing for racking strength and an EWI system externally seems to be a robust and efficient approach. I’d be keen to hear views on this and any lessons learned.
Roof build-ups
Whether flat or pitched, are there recommended best-practice details for insulation, airtightness, and moisture control?
Building in masonry is still an option, but one reason we’re leaning towards timber frame is that my 16-year-old son is currently doing a carpentry apprenticeship, and this feels like a great opportunity for him to be involved and develop his skills.
If anyone can point me towards online resources, technical details, drawings, or case studies covering slab, wall, and roof best practice—or share their own experience with a similar project—I’d be extremely grateful.
For roofing finishes, if we go pitched I’d likely consider Eternit / fibre-cement slates, possibly aluminium-clad profile sheets, or other lightweight profiled systems.
Thanks in advance for any advice or pointers—really appreciate the collective knowledge on this forum.

If you are undecided re whether to add a 2nd storey why not initially flat roof with no roof-lights? If you stick with 1 storey you can add roof-lights, or if you add a 2nd storey you can just bolt it on top of the exg. Why not frame the flat roof ('first floor') with a filled-in stairwell?

Accidental duplicate deleted

Posted By: Nick ParsonsWhy not frame the flat roof ('first floor') with a temporarily-filled-in stairwell?

Unless access to the 2nd story of the extension is through one of the existing rooms of the house (typically through an existing window) which avoids a second staircase in the house.

P-in-H wrote:

''Unless access to the 2nd story of the extension is through one of the existing rooms of the house (typically through an existing window) which avoids a second staircase in the house.''

Indeed! Typing with my brain switched off! (Though 2 staircases can be fun if you have kids!)

Posted By: Victorianecois there any benefit over engineering 1/5 of the footprint using PHPP?

Using PHPP doesn't commit you to over engineering, but it is a better thermal modelling tool than most.

Rooflights are bad from a thermal perspective, particularly if they're shaded by the house and only see north light. Triple glazed windows might well be worth while if north-facing. Maybe worth designing in some external shutters.

Gareth, if you want your son to gain experience then stick building will allow him to learn more, I'd think.

Hilliard produced quite a complicated looking design for our slab, that varies in thickness over its area. There's rebar throughout to give tensile strength and it's thicker where there is load on top of it, plus in strenghening ribs. As PiH says, the heavily loaded parts use EPS 300 and the rest uses EPS 90 (pretty similar to EPS 100). Your structural engineer will decide what's needed where for you.

Posted By: Peter_in_HungaryI have a bit of difficulty in seeing how a pitched roof would work with single story extension on a 2 story house.

Presumably a monopitch sloping down away from the house.

A monopitch roof makes sense initially and would also be better in terms of weather resistance compared to flat roofing it would also give me a higher ceiling height at one end of the room which I believe would give a grander illusion of space.

However would there then be a way to make benefit from both morning son from the East and as the sun passes around the property from the west in that case we're to make sense to have a dual pitch roof sloping from left to right as opposed to a single monitor roof sloping from the house toward the garden full stop or would of course the values or roof lights in the single monopitch roof wood fulfill this purpose and anyway

Would a 25% total glazed area in this room makes sense but would also like to bring the garden into the living area so these are strong considerations to take on board.