Most lofts in Barnet and Hertfordshire can be converted. The real questions are which type of conversion fits your roof, whether you have enough height, and whether you need full planning permission or can build under permitted development. Here is how to work that out before you pay anyone for drawings.
How to Tell if Your Loft Is Suitable
A builder or surveyor will confirm it on site, but you can rule most lofts in or out yourself in ten minutes with a torch and a tape measure. Four things decide it.
1. Head height
Measure from the top of a ceiling joist straight up to the underside of the ridge timber at the highest point. This raw figure is the single best predictor. Over about 2.4m is comfortable: once the new floor and insulation go in, you keep good standing height across the middle of the room. Between 2.2m and 2.4m is usually workable but tight, and the design has to work harder. Under about 2.2m does not rule the loft out, but the realistic options become raising the roof (which normally needs full planning) or lowering the ceilings of the rooms below, both of which add cost and time.
Building regulations do not set a minimum ceiling height for the room itself, but they do require headroom over the new stair (2m, with a relaxation for loft conversions to 1.9m at the centre of the stair), and a room you can only stand up in at one point is not much of a room. A dormer adds full height over most of the floor, which is why it is the usual answer for lofts in the 2.2m to 2.4m range.
2. Roof structure: cut rafters or trusses
Houses built before the 1960s usually have a traditional cut roof: individual rafters, supported by horizontal purlins and props, with the loft largely open. These are the easiest to convert, because the structure can be reworked around new steel beams without much fuss.
From the 1960s onwards, many houses were built with prefabricated trussed rafters: triangular frames with diagonal timbers (webs) running through the loft every 600mm or so. A trussed roof can still be converted, but the webs are part of the structure, so they cannot simply be cut out. A structural engineer designs replacement support, usually new steel beams and a new floor, which adds to the cost. If your loft looks like a forest of W-shaped timbers, expect the engineering to be a bigger part of the price.
3. Water tanks, chimneys and services
A cold water tank in the loft is common in older houses. It is normally removed and the hot water system changed (a combination boiler or a pressurised cylinder elsewhere), which needs planning into the budget. Chimney stacks and breasts that run through the loft take up floor space; removing them needs an engineer's design and building control approval, and on a shared chimney it brings in the Party Wall Act. Old wiring and pipework in the loft floor will be rerouted when the new floor goes in.
4. Where the stair goes
The best loft conversions are often decided on the landing below, not in the loft. The new stair usually runs over the existing one, so the layout of the first floor matters: you want it to land in the loft where there is full head height, without swallowing too much of a bedroom underneath. In a bungalow, the stair takes ground-floor space instead. Building regulations also need a protected route down to the front door and mains-wired, interlinked smoke alarms, which can affect open-plan ground floors.
Loft Conversion Types Compared
These are the typical all-in price bands we use across the site. Your quote will depend on size, roof structure, finish and access, and you should always check whether VAT is included: it is the commonest omission in loft quotes. The loft conversion cost guide has the full breakdown.
| Type | Typical cost | Best for | Usual planning route |
|---|---|---|---|
| Velux / rooflight | £18,000–£30,000 | Lofts with generous height already; offices, playrooms, smaller bedrooms | Usually permitted development |
| Rear dormer | £30,000–£50,000 | Gable-end semis, terraces and bungalows; a double bedroom with en-suite | Usually permitted development |
| Hip-to-gable | £35,000–£55,000 | Hipped-roof semis, detached houses and bungalows | Often permitted development |
| Hip-to-gable + rear dormer | £45,000–£65,000 | Hipped semis that want a full-width, full-height floor | Often permitted development |
| L-shaped dormer | £40,000–£60,000 | Victorian and Edwardian terraces with a rear back addition; two rooms | Planning likely |
| Mansard | £50,000–£75,000+ | Terraces, low lofts and flat roofs; maximum space | Full planning required |
If you are adding a rear extension at the same time, price it separately and add the two: a single-storey rear extension typically runs £45,000–£75,000 on its own.
Planning: Barnet vs Hertfordshire
Most rear dormers, hip-to-gables and rooflight conversions on houses fall under permitted development. The national limits are up to 40 cubic metres of additional roof space on a terrace and 50 cubic metres on other houses, nothing higher than the existing ridge, no extension on the roof slope facing the road, and materials that match the house. Flats and maisonettes have no permitted development rights, and conservation areas and Article 4 directions remove them for roof extensions. Even when you are within the rules, we recommend a Lawful Development Certificate: it is the council confirming the work is lawful, and it is the document a buyer's solicitor will ask for when you sell.
In Barnet, one council handles the whole borough. The pressure points are its many conservation areas, where roof extensions need a full application, and Hampstead Garden Suburb, where the Suburb Trust's own consent is needed on top of the council's. Rear dormers are usually fine outside those areas; front and side roof changes are where applications get harder. Our Barnet loft planning guide covers the local detail, and the loft planning permission guide explains the underlying rules.
In Hertfordshire, planning is decided by the district or borough council, not the county: St Albans, Hertsmere, Dacorum, Three Rivers, Watford, Welwyn Hatfield, East Herts, North Herts and others. Each has its own conservation areas and some have Article 4 directions on particular streets. Green Belt villages are judged against the size of the original house, and in Letchworth Garden City the Heritage Foundation's consent is needed alongside the council's. The Hertfordshire planning permission guide lists which council covers each town.
Whichever route applies, building regulations approval is separate and always needed for a loft conversion. If you are not sure whether your street is in a conservation area, the conservation areas guide explains how to check, and terraces and semis should read the party wall guide early.
How Long a Loft Conversion Takes
Allow roughly 6 to 12 weeks before work starts for a permitted development loft: measured survey, drawings, structural calculations, a Lawful Development Certificate and party wall notices, which need two months' notice before work on a shared wall. If you need full planning, the council's target is 8 weeks from validation, so allow three to five months. On site, a rear dormer typically takes 8 to 10 weeks, a hip-to-gable 8 to 12 weeks and a mansard 12 to 16 weeks. The loft conversion process guide walks through each stage.
Frequently Asked Questions
How much head height do I need for a loft conversion?
Measure from the top of a ceiling joist to the underside of the ridge at the highest point. Over about 2.4m is comfortable. Between 2.2m and 2.4m is usually workable with careful design, often with a dormer. Under about 2.2m usually means raising the roof or lowering the ceilings below, which costs more and normally needs planning.
Can every loft be converted?
Most lofts on houses can be converted in some form, but not every one is worth it. Very low lofts, shallow roof pitches and heavily trussed roofs push the cost up, and flats, listed buildings and conservation areas need full consent. A survey and a structural engineer confirm what is possible.
Do I need planning permission for a loft conversion?
Often not. Most rear dormers, hip-to-gables and rooflight conversions on houses are permitted development, within 40 cubic metres on a terrace and 50 on other houses, with nothing on the roof slope facing the road. Mansards, flats, conservation areas and streets with Article 4 directions need a full planning application.
What is a Lawful Development Certificate and do I need one?
It is the council formally confirming that work under permitted development is lawful. It is not compulsory, but we recommend getting one before work starts, because it is the document a buyer's solicitor will ask for when you sell. The council's target for a decision is 8 weeks.
Can a loft with roof trusses be converted?
Yes, usually. Trussed rafters, common in houses built from the 1960s onwards, cannot simply be cut out because the diagonal timbers are part of the structure. A structural engineer designs replacement support, normally new steel beams and a new floor, which adds to the cost compared with a traditional cut-rafter roof.
How long does a loft conversion take?
Allow about 6 to 12 weeks for design, structural calculations and a Lawful Development Certificate, or three to five months if you need full planning. On site, a rear dormer typically takes 8 to 10 weeks, a hip-to-gable 8 to 12 weeks and a mansard 12 to 16 weeks.