Adding more floor area is not always the same thing as making a home work better. Sawtooth House in Kingston upon Thames is a useful example of the difference: Francesco Pierazzi Architects added just 15m², but used the project to rethink much more of the existing Victorian house.

Published project information records 125m² of existing accommodation being reworked and an overall finished area of 180m². The extension's most obvious feature is its double sawtooth roof, but the stronger lesson for homeowners is less about copying its shape and more about what the roof, layout and retrofit are doing together.

If you are planning an extension, the project suggests a better starting question than “how much can we add?”: which parts of the current house are failing, and how can new space make the old space work harder too?

A rear extension can solve one problem while creating another. Push a Victorian house further into the garden and the new room may be bright, but the middle of the original ground floor can end up darker and more disconnected.

Sawtooth House avoids treating the extension as a self-contained box. The modest addition sits within a wider reorganisation of the home, so circulation, daylight and the relationship between old and new rooms are considered together.

That is a useful discipline before a homeowner settles on an extension size. Draw the existing plan and mark the problems before drawing the proposed one. Is the kitchen too small, or simply in the wrong place? Is the route from the front door to the garden awkward? Does storage pinch a circulation route? Are there rooms that technically have enough floor area but are hard to furnish or poorly lit?

Those questions can change the brief. Sometimes another metre of depth is valuable. Sometimes moving a doorway, opening a sightline or repurposing part of the existing plan does more for everyday life than simply increasing the footprint.

In this project, the 15m² addition matters because it unlocks a larger spatial change. That is a much more useful benchmark than square metres alone.

The sawtooth roof references Kingston's industrial past, including the area's former manufacturing buildings. Visually, it gives the extension its identity. Practically, it provides opportunities to bring daylight into the plan from above.

RIBA Journal reports that the rooflights are angled to catch eastern light in the morning and warmer western light later in the day. Architecture Today also notes that the roof draws light into the house while creating different internal volumes.

For a typical rear extension, that principle is worth borrowing even if the roof shape is not.

Large doors to the garden can make the back of a new room feel bright without doing much for the old dining room, hallway or sitting room behind it. Rooflights, clerestory glazing or a carefully placed internal opening can help daylight reach the part of the house that becomes furthest from an external wall.

The useful design question is therefore not “how much glass can we fit at the back?” but “where will the darkest point of this plan be once the extension is built?”

Thinking about that early can influence roof form, ceiling height, the position of internal walls and even where tall kitchen units are allowed to go.

The extension also shows why circulation deserves as much attention as room size.

When assessing a proposed plan, trace the journeys that happen every day: carrying shopping to the kitchen, moving from cooking to the dining table, taking laundry through the house, letting children move between inside and garden, or getting from the front door to a downstairs WC without cutting through furniture.

Then test the plan with doors and appliances open.

A route that looks generous on a drawing can narrow quickly once an oven door, dishwasher, dining chair or cupboard is in use. Conversely, a modest extension can feel much more spacious if it removes pinch points and lets several routes overlap comfortably.

Sawtooth House's changing ceiling heights also help distinguish areas within an open-plan space. That is another idea worth borrowing. Open plan does not have to mean one undifferentiated room. Changes in ceiling height, floor material, lighting, joinery or furniture can create zones without rebuilding solid walls.

Externally, the extension uses brick, which gives it a material relationship with the Victorian house while allowing the new work to remain visibly contemporary. Inside, oak joinery, pastel colours and a multicoloured terrazzo surface make the addition more playful.

For homeowners, the lesson is that “matching” does not have to mean imitation.

A new extension can relate to an old house through material, colour, proportion or rhythm while still being clearly of its time. Trying to reproduce historic brickwork or detailing exactly can be expensive and may still look obviously new. In other cases — particularly listed buildings or sensitive conservation settings — a closer match may be required.

The right answer depends on the house and its planning context, but deciding deliberately is better than ending up with a halfway treatment that neither blends nor contrasts convincingly.

Published project information says the existing house received additional insulation and airtightness improvements, while heating and hot water were moved to an air-source heat pump. Mechanical ventilation with heat recovery was installed, and a 7,500-litre underground rainwater tank supplies garden irrigation.

Those measures should not be copied as a standard specification for every extension. Heat pumps need to be designed around the building's heat loss, emitters and controls. MVHR is a whole-house ventilation strategy that depends on good design, duct routes and airtightness. A rainwater system needs space, drainage planning, maintenance and a use case that justifies it.

The transferable point is timing.

A major renovation may expose floors, ceilings, walls and service routes that are expensive to disturb again later. If you already expect substantial work, it is sensible to assess insulation, heating, ventilation, hot water, electrics and water use before the final layout and finishes are fixed.

That does not mean doing every upgrade at once. It can mean making sure today's project does not make tomorrow's work unnecessarily difficult.

Before deciding how far to extend, ask:

1. What is actually wrong with the existing plan — lack of area, poor circulation, weak daylight, storage or all four?
2. Which existing rooms should work better because of the extension?
3. Where will daylight be weakest once the house becomes deeper?
4. What happens to everyday routes when doors, chairs and appliances are in use?
5. Should the new work blend with the old house, deliberately contrast with it, or use a controlled mix of both?
6. Which fabric and services decisions are easier to make while the house is already being opened up?

These questions will not produce a Sawtooth House — nor should they. They are useful because they move the conversation away from copying a distinctive project and towards understanding why it works.

The 15m² figure is what makes Sawtooth House attention-grabbing. The more valuable idea is that the designers did not ask those 15 square metres to solve the house on their own. The extension became a catalyst for improving the rooms, light and systems around it.