Kitchen Extensions

What people actually want

Not a bigger kitchen. A different relationship between the kitchen and everything else.

Almost nobody extends because they need more worktop. They extend because the kitchen is a separate room at the back of the house, the family is somewhere else, and whoever is cooking is on their own with a wall between them and everyone they live with.

What solves that is not floor area, it is removing the wall. The extension provides the space for the room to become a kitchen, a dining table and a sitting area at once, and the structural opening into the existing house is what makes the whole ground floor read as one place rather than three.

That distinction matters because it changes what you should spend money on. A modest extension with a wide, properly formed opening into the existing rooms is a better house than a large extension reached through a doorway. If the budget is tight, the opening is not the place to save.

The structural opening into the house

The bit that turns two rooms into one, and the bit that has to be calculated.

Removing the rear wall of the house is the structural event of a kitchen extension. That wall is carrying floor joists and often the roof, and the load has to go into a steel beam, the beam into padstones, and the padstones onto masonry capable of taking a concentrated point load it was never designed for.

The engineer sizes all three. Where the masonry below a bearing is not adequate, the load has to be taken down to foundation level, which sometimes means a pier or a new pad inside the existing footprint.

Everybody wants the widest possible opening with the slimmest possible beam. Beam depth is a function of span and load and there is no negotiating with it. On a wide span you can sometimes hide a deeper steel within the floor zone above, or use two beams, or change the structural arrangement. What you cannot do is ask for a smaller steel.

The related decision is whether to keep a downstand. A beam expressed as a shallow downstand across the opening is often the honest answer on a wide span, and it reads as a deliberate threshold between the old house and the new room rather than as a compromise. Trying to hide a large steel entirely can push the floor structure around in ways that cost more than the visual gain is worth.

Floors, levels and thresholds

The floor is poured once and lived on for thirty years. It is worth thinking about.

A new extension has a new floor, and matching its finished level to the existing house is a decision made at slab stage, not at flooring stage. Get it wrong and you have a step between the old kitchen and the new room, which on an open plan ground floor is both a trip hazard and a permanent reminder that the space is two rooms pretending to be one.

The build up matters. Under the slab goes a compacted sub base, a damp proof membrane, then insulation, then the concrete. If underfloor heating is going in, the pipes sit in or on the screed above that. Every one of those layers has a thickness, and they add up to more than people expect. Working backwards from the finished floor level of the existing house, through the flooring you intend to lay, is the only way the levels come out right.

Existing houses are rarely level anyway. A Victorian terrace floor can fall twenty or thirty millimetres across a room. The new slab is poured flat, so the junction has to be resolved somewhere, and it is better resolved deliberately than discovered by a floor fitter.

Level thresholds to the garden are achievable and they genuinely change how the room is used. They also need designing: a channel drain outside the doors, a properly detailed upstand and damp proof course, and a drainage route for the channel. Done properly they are excellent. Done as an afterthought they are the most reliable way to get water into a new floor.

The flooring choice interacts with heating. Large format porcelain and engineered oak both work over underfloor heating. Solid timber is more temperamental. Decide the flooring before the screed goes down, because the depth changes.

What a kitchen extension costs

Two numbers, and people routinely conflate them.

There is the cost of the extension, and there is the cost of the kitchen. They are separate, they are both substantial, and a builder’s quotation almost never includes the second.

The building work, indicative and current at the review date below: a single storey kitchen extension on reasonable ground with decent access typically runs £2,200 to £3,000 per square metre for the shell and a standard finish. On a 20 square metre extension that is roughly £44,000 to £60,000. Deep foundations, difficult access, wide structural openings and extensive glazing push above that.

That includes groundworks and foundations, drainage connections and alterations, structure and the steel, external walls and insulation, roof, windows and external doors, plastering, first and second fix carpentry, electrics and plumbing to a standard specification, and making good.

It excludes, and you should budget separately for: VAT, the kitchen units, worktops and appliances, flooring, decoration, professional fees for drawings and structural calculations, planning and Building Control fees, party wall surveyor fees where a neighbour dissents, and service diversions.

The kitchen itself is a genuinely open ended number. A well made mid range kitchen with quartz worktops and decent appliances for a room of this size is commonly £15,000 to £30,000 installed. It is entirely possible to spend three times that and entirely possible to spend half. Decide the rough figure early, because it affects how much of the total budget is available for the building.

Heating an open plan kitchen

A room that is half glass and open to the rest of the house behaves differently to the kitchen it replaced.

The old kitchen was a small room with a door and one radiator. The new one is a large volume with a large area of glazing, open to the hall or the living room, and it needs thinking about at both ends of the year.

Underfloor heating is the usual answer and it suits these rooms well. It frees the wall space a radiator would take in a room where the walls are units and glass, it heats a large floor plate evenly, and it is comfortable underfoot on a tiled floor. It is slower to respond than a radiator, which matters less in a room that is occupied for long stretches. It goes in before the screed, so it is decided early or not at all.

The system needs zoning. An open plan kitchen diner on the same zone as the rest of the ground floor will be either too warm or too cold, because it has a completely different heat loss profile to a small front room. A separate zone with its own thermostat is a small cost at first fix and the difference between a room that is comfortable and one that is fought with.

Check what the extension does to the boiler. Adding twenty or thirty square metres of heated space, some of it with a large glazed area, increases the load. On an older system that is worth a proper assessment rather than an assumption, and replacing or upgrading while the walls are open is considerably cheaper than afterwards.

At the other end of the year, overheating is the more common complaint in these rooms. Cooking adds heat, glazing adds heat, and a flat roof over a south facing wall of doors will make the space unusable on a hot afternoon without solar control glazing and cross ventilation designed in. An opening rooflight on the opposite side of the room from the doors is the cheapest and most effective thing you can do, because it lets hot air out at the top while cool air comes in low.

Which houses in Colchester suit which approach

The housing stock decides the shape of the kitchen extension more than taste does.

Victorian and Edwardian terraces, through New Town and the streets off Maldon Road, have a rear outrigger with a narrow alley beside it. The kitchen is usually in the outrigger, dark and only a couple of metres wide. The answer here is nearly always a side return: build out into that alley, take out the outrigger’s side wall, and the kitchen becomes the full width of the plot. Light comes from a run of rooflights along the boundary, because a boundary wall a metre away gives you nothing through a vertical window. Access is the cost driver, since everything comes through the house.

Interwar and 1930s semis, which is most of Lexden, Prettygate and older Shrub End, have side access and a decent garden depth. A single storey rear extension has room to work without swallowing the garden, and the side gate makes the build materially cheaper. These houses usually have a separate dining room, so the opportunity is to take out two walls rather than one and get a genuinely large open plan space. Watch the middle room: extending the full width leaves it without a window.

Post-war and 1960s housing around Greenstead, Monkwick and Old Heath has generous plots and simple rectangular footprints, detached and semi detached alike. Structurally the easiest of the lot, and there is often room to go both back and sideways, which gives you an L shaped kitchen and dining space with light from two directions.

Estate housing from the 1980s onwards, across Stanway, Myland and Highwoods, is simple to build on but the plots are tight and the boundaries close. The constraint is rarely structural and frequently legal: restrictive covenants on the title, or a management company whose consent is needed, and neither of those is something the council checks. We look at the deeds early on these.

Village property around Dedham, Boxted and Layer de la Haye brings conservation areas and older construction, sometimes with no meaningful foundation under the existing rear wall at all. The design conversation is longer and the answer is usually a more considered one, but the results in these houses are the best on our patch.