Home improvement glossary

Plain-English meanings of the terms you'll meet in quotes, drawings and building control letters.

Approved Documents

Government guidance documents, each identified by a letter, that explain how to meet the Building Regulations in England. For example, Part A covers structure, Part B fire safety, Part F ventilation, Part L energy efficiency and Part P electrical safety.

Article 4 direction

An order made by a council that removes some or all permitted development rights in a defined area, often a conservation area. Where one applies, work such as replacing windows, changing a front door or extending can need a planning application.

Asbestos survey

An inspection by a qualified surveyor, usually with samples sent for testing, to find any asbestos before building work disturbs it. HSE says asbestos may be present in any building built or refurbished before 2000, including textured coatings, soffits and old floor tiles.

Bifold doors

Glazed doors made of several panels that fold back against each other on a track, opening most of the width of a wall to the garden. Like other glazed external doors, replacements must meet Part L energy limits and safety glazing rules.

Build over agreement

Written permission from the sewerage company to build over, or within 3 m of, a public sewer. It usually involves checking the pipe's position and condition, sometimes with a CCTV survey, and may affect the depth and design of the foundations.

Building control body

The organisation that checks building work against the Building Regulations. It can be your council's building control service or a private registered building control approver. It reviews plans, inspects the work at key stages and issues the final certificate.

Building notice

A quick way to tell building control about smaller, simpler work without submitting detailed plans first. Work can start two days after the notice. The risk is that anything found not to comply has to be put right after it has been built.

CERTASS

One of the government-authorised competent person schemes for replacement windows and doors in England. Installers registered with it can certify that their work meets the Building Regulations and register it with building control, and the homeowner receives a certificate.

Competent person scheme

A government-authorised scheme that lets registered installers certify their own work as meeting the Building Regulations, without a separate building control application. Schemes cover work such as windows, electrics, heating and roofing. You should receive a certificate within 30 days of completion.

Completion certificate

The certificate building control issues when it is satisfied that finished work meets the Building Regulations. A private approver issues a final certificate instead. Keep it with your property documents, because a buyer's solicitor will usually ask for it.

Conservation area

An area the council has designated for its special architectural or historic interest. Permitted development rights are reduced there, so some extensions, roof alterations and external cladding need planning permission, and work to trees usually needs notice to the council first.

Damp-proof course (DPC)

A layer of waterproof material built into a wall, normally at least 150 mm above outside ground level, to stop moisture rising from the ground. Paths, driveways and soil should be kept below it so rain can't splash or bridge over it.

Damp-proof membrane (DPM)

A heavy-duty plastic sheet laid under or within a concrete ground floor to stop moisture coming up from the ground. In an extension it is lapped into the damp-proof course of the walls so the whole floor and wall junction stays dry.

Dormer

A box-shaped structure built out from a sloping roof, with vertical walls, a window and usually a flat roof. A dormer adds full-height floor space to a loft conversion, and a full-width rear dormer is common on semi-detached and terraced houses.

Dropped kerb

A lowered section of kerb and strengthened pavement that lets a vehicle cross from the road to a driveway. You must apply to your council, which inspects the site and sets the specification. Planning permission is also needed on classified roads.

EPDM

Short for ethylene propylene diene monomer, a synthetic rubber membrane used to cover flat roofs. It is usually laid as a single sheet bonded to the roof deck, which means few or no joints, and is finished with trims at the edges.

Fascias and soffits

The fascia is the board along the edge of the roof that the gutter fixes to. The soffit is the horizontal board underneath the overhang. In older homes these boards can contain asbestos, so they should be checked before they are removed.

FENSA

One of the government-authorised competent person schemes for replacement windows and doors in England. Registered installers certify that the work meets the Building Regulations and register it with building control. You receive a certificate, which buyers' solicitors usually ask to see.

First fix and second fix

First fix is the work done before walls and floors are closed up, such as running cables, pipes and back boxes. Second fix comes after plastering and covers connecting sockets, switches, taps, radiators, sanitaryware, doors and skirting.

Full plans application

A Building Regulations application with detailed drawings and specifications, checked before work starts. The building control body has five weeks to decide, or two months if you agree. It gives you formal approval, so you know the design complies before it is built.

Gas Safe Register

The official list of engineers legally allowed to work on gas appliances and pipework in homes. Anyone fitting or moving a gas hob, cooker, boiler or gas pipe must be registered. Their ID card shows the types of gas work they are qualified for.

GRP roofing

Glass-reinforced plastic, often called fibreglass. A GRP flat roof is built up from resin and glass-fibre matting laid over timber boards, then finished with a coloured topcoat. It sets into a hard, joint-free surface and suits flat roofs and dormers.

Hip-to-gable

A loft conversion that replaces a sloping side of a hipped roof with a vertical gable wall and extends the ridge across. It greatly increases loft space on semi-detached and detached houses with hipped roofs, and is often combined with a rear dormer.

Lawful development certificate (LDC)

A legal document from the council confirming that a proposed or existing project is lawful, for example that an extension falls within permitted development. It isn't compulsory, but it is useful proof when you sell. From April 2026 the fee for proposed work is £274.

Lintel

A beam built in over a window or door opening to carry the brickwork or blockwork above it. Lintels are usually steel or concrete. Making a wall opening wider normally means fitting a longer, stronger lintel or a steel beam designed by an engineer.

Mansard

A loft conversion that replaces a roof slope with a near-vertical wall, at around 70 degrees, topped by a flat roof, with windows set into small dormers. It gives the most usable space of any loft type but usually needs planning permission.

Padstone

A solid block of concrete or engineering brick set into a wall under the end of a steel beam. It spreads the load from the beam into the wall below so the masonry isn't crushed. A structural engineer specifies its size.

Part P

The part of the Building Regulations that covers electrical safety in homes in England. Fitting a new circuit, replacing a consumer unit or altering circuits around a bath or shower must be certified by a registered electrician or approved by building control.

Party wall

A wall that stands across the boundary between two properties and is shared, such as the wall between semi-detached or terraced houses. Masonry garden walls on the boundary are also covered, but wooden fences are not. Work affecting one is controlled by the Party Wall etc. Act 1996.

Party wall award

A legal document drawn up by party wall surveyors when a neighbour doesn't consent to notified work. It sets out what will be done, how and when, usually includes a schedule of condition, and says who pays. The building owner normally pays the fees.

Party wall notice

Written notice that a homeowner must give neighbours before certain work under the Party Wall etc. Act 1996. That means two months for work to a party wall, and one month for a new boundary wall or for digging foundations close to a neighbour's building.

Permeable paving

A driveway or path surface that lets rainwater soak through into the ground, such as permeable block paving, gravel or resin-bound stone on a suitable base. In England, front gardens paved over 5 m² must be permeable, or drain to a permeable area, to avoid needing planning permission.

Permitted development (PD) rights

A national planning permission that allows certain household work, such as many single-storey rear extensions and loft dormers, without applying to the council, provided strict limits on size, height and position are met. Flats and some areas don't have the same rights.

Prior approval

A light-touch council check needed for some permitted development, including larger single-storey rear extensions of up to 6 m (or 8 m on a detached house). Neighbours get at least 21 days to comment and the council has 42 days to decide.

Provisional sum

An allowance in a quote for work that can't be priced exactly until it is uncovered, such as drain repairs or replacing hidden timbers. It should be clearly labelled, and it is adjusted up or down to the actual cost once the work is known.

Regularisation

A council process for approving building work that was carried out without Building Regulations consent, for work done after 11 November 1985. You may need to open up parts of the work for inspection and pay for alterations before a regularisation certificate is issued.

Render

A coating applied to external walls to protect and finish them. Traditional sand and cement render is usually painted. Modern through-coloured renders, including silicone and monocouche types, don't normally need painting. In conservation areas, rendering may need planning permission.

Roof light

A window set into the slope of a roof, often called a Velux after one well-known brand. A roof light conversion adds them to a loft without changing the roof shape. They must not project more than 150 mm from the roof slope to stay permitted development.

RSJ (steel beam)

Short for rolled steel joist, the name builders use for the steel beams that carry walls and floors over a new opening, such as a knock-through or the back wall of an extension. A structural engineer sizes the beam and specifies how it is supported.

Schedule of condition

A dated written and photographic record of the condition of a neighbour's property, or your own, before building work starts. It isn't a legal requirement, but it makes any later claim about cracks or damage far easier to settle fairly.

Screed

A layer of sand and cement, or a flowing anhydrite mix, laid over a concrete slab or insulation to give a flat, level surface for the final floor. Underfloor heating pipes are often set into it. It needs time to dry before flooring goes down.

Skim coat

A thin finishing coat of plaster, usually only a few millimetres thick, applied over plasterboard or existing plaster to give a smooth surface ready for decorating. New skim needs to dry fully and be sealed with a mist coat before painting.

Snagging

The final check of finished work, when small defects such as paint marks, sticking doors, gaps in sealant or loose trims are listed and put right. Walking round together with a written snagging list before the final payment helps everyone agree when the job is complete.

Soakaway

An underground pit, filled with rubble or plastic crates, that collects rainwater from roofs or driveways and lets it soak gradually into the ground. It needs ground that drains well, which is usually checked with a percolation test before it is designed.

Structural calculations

An engineer's calculations showing that beams, lintels, foundations and other load-bearing parts are strong enough for the loads they carry. Building control normally needs them before approving extensions, loft conversions and knock-throughs, and they set the sizes of the steelwork.

Sub-base

The compacted layer of crushed stone under a driveway, patio or path that carries the weight and spreads it into the ground. Most driveway failures start here. Permeable driveways use an open-graded stone sub-base that lets water drain through.

Tanking

A waterproof membrane, sheet or liquid system applied to walls and floors before tiling in wet areas such as showers and wet rooms. Tiles and grout aren't waterproof on their own. British Standard BS 5385-1 recommends waterproofing wet areas in homes.

Trickle vent

A small adjustable vent, usually in the top of a window frame, that lets a little fresh air in while the window is shut. If your old windows had them, replacement windows must too, and they help control condensation and mould in airtight homes.

U-value

A measure of how quickly heat passes through part of a building, such as a wall, roof or window, in watts per square metre per degree (W/m²K). The lower the number, the better the insulation. Replacement windows in England must achieve 1.4 W/m²K or better, or energy rating band B.

Underpinning

Strengthening an existing foundation by extending it deeper, or onto firmer ground, usually because of subsidence or to support new loads. It needs Building Regulations approval and an engineer's design, and if it involves a party wall, a party wall notice too.